The world’s population is ageing. According to the United Nations ^[1], at the global level, approximately 9% of people were aged 65 or over in 2019, and the proportion is expected to reach nearly 12% in 2030, 16% in 2050, and 23% by 2100. Especially in developed counties, the ageing populations are an established trend. For example, in 2019, 18% of the population in Europe and North America was 65 and older, followed by 16% in Australia and New Zealand. According to current projections, one out of four Europeans and North Americans could be 65 or older by 2050. As one of the most significant social changes of the twenty-first century, the ageing of the population will affect nearly every sector of society, including the labour and financial markets, housing, transportation, social security, as well as family structures and intergenerational relationships ^[2].

Along with the established trend of population ageing, most older people prefer to age in place, which means staying at home in the community. In particular, with the baby boomer cohort having reached 60 years of age in 2006, many of those who were born from the mid-940s to the mid-1960s are now fuelling a growing demand for ageing in place ^[3] and the corresponding support and care required. Unfortunately, ageing in place is often challenging for older adults because of a variety of common barriers, including diminished physical abilities, the rising costs of long term care, the increasing risk of social isolation, and a lack of preparedness in the community environment ^[4]. In particular, the effects of social isolation and loneliness on the health and wellbeing of older people have been demonstrated ^[5].

In order to better facilitate ageing in place, naturally occurring retirement communities (NORCs), a concept that originated in the United States of America in the 1980s, have emerged as a model of collaborative care that can support older people, allowing them to remain in their homes as long as possible and avoid a shift to more restrictive environments ^[6]. A NORC is a neighbourhood or building complex that was not originally designed for older adults but eventually came to accommodate a large percentage of older residents. While not initially created to help older adults age in a community, NORCs have evolved naturally and provide a way for older adults to live independently ^[7]. Furthermore, NORC supportive service programs (NORC-SSPs) have been implemented and developed for more than 30 years; they have improved the physical and psychological health of the older participants, increased the efficiency of resource allocation from service providers, enhanced funding support from government and related organisations, and promoted the establishment of more beneficial ageing policies by policy makers [8,9]^[8][9].

Despite the great potential of NORCs to facilitate ageing in place, they have not attracted much research attention. On the one hand, although NORCs are already a factual phenomenon globally, there is limited research on NORCs, with most of the existing studies conducted in the U.S., where NORCs originated. On the other hand, the majority of prior studies concentrated on one specific area of interest, such as the social network of older adults, the relationship between service providers and older residents, and older individuals’ participation in program activities. What is lacking is a comprehensive review of previous NORC studies in order to form a holistic picture of how NORCs support successful ageing in place.

2. Ageing in Place

Ageing in place is defined as “the ability to live in one’s own home and community safely, independently, and comfortably, regardless of age, income, or ability level” ^[10]. As independent living may be a better option for the older population and for society compared to assisted living, such as nursing homes ^[11], the aim of many ageing-in-place programs is to enable older persons to remain independent in communities where social networks of family and friends have been established. It has been demonstrated that ageing in place leads to many positive outcomes. The social sector offers opportunities for preventing premature institutionalisation, delaying the demand for costly health services, creating efficiencies of scale in service delivery, and increasing possibilities for community involvement, volunteerism, and leadership ^[12]. At the individual level, retirement in place can improve self-efficacy, provide social support within the community, and allow you to maintain a sense of familiarity and belonging ^[13]. Furthermore, cognitive functions, daily life activities, and depression have also been reported as improved ^[14]. However, as individuals’ lives change, so do the environment and policies that may affect their residence over time ^[15]. Lau and Scandrett ^[16] identified three types of barriers to ageing in place: individual, community, and social. The barriers at the individual level call for preventing diseases that may cause disabilities (e.g., limitations on daily living activities) ^[17], maintaining social ties (e.g., family and neighbourhood) ^[18], and modifying the home/living environment in a timely manner ^[19]. At the community level, services that cater to the social and health needs of vulnerable older people are essential ^[20]. Finally, at the societal level, providing adequate resources to each community’s older residents requires public assistance ^[21]. Similarly, Cutchin ^[22] identifies managing instability or irregular changes in the individual’s circumstances and needs as a major obstacle to successful ageing in place. Individuals, local communities, and society must make concerted efforts to enable those with disabilities to perform necessary home renovations and live in their current home ^[16]. As a matter of fact, a 2011 study found that only 18% of households had lived in the same house for 20 years or more, despite their strong desire to do so ^[23]. Taking this into account, communities such as NORCs can leverage coordinated efforts among their members to facilitate ageing in place, and this deserves further study ^[24].

3. Definition of Naturally Occurring Retirement Community (NORC)

A naturally occurring retirement community (NORC) describes a community that was not designed for the needs of older people but has a significant proportion of senior residents due to natural migration patterns ^[25]. The attractions of NORCs include both neighbourhood services that support older people’s needs and capabilities as well as safety and close proximity to age peers. Because older people are concentrated in geographically close areas, it is possible to serve them effectively and facilitate formal and informal cooperation among residents, communities, service providers, and the public sector. Therefore, in order to facilitate the physical and psychological wellbeing of older people, the NORC is viewed as a critical model for ageing ^[26]. However, very few studies have tried to clearly define the concept of a NORC. Usually, studies that define NORCs agree on the composition, but the details differ. It is generally believed that a NORC is a geographical area in which a large proportion of older residents live in a specific area or in housing that was not designed or planned for the older people at the beginning. However, what constitutes a “large proportion” of the population and at what age a person should be included in that proportion are not agreed upon. Hunt and Gunter-Hunt ^[25] first defined the term “Naturally Occurring Retirement Community” as “a housing development that is not planned or designed for older people, but which over time comes to house largely older people”. In addition, they mentioned that NORCs may vary considerably in scale. For example, NORCs can range from a local neighbourhood with a disproportionate number of older residents to an apartment building or complex. According to Hunt and Ross ^[27], a NORC is a type of housing development that does not plan or design for older people but has a significant proportion (over 50%) of residents at least 60 years old. Since apartments were the most common form of alternative housing for older people in the U.S. at the time, the authors focused on apartment NORCs, despite NORCs having many different forms. Having at least 40% of household heads who are 65 or older in a census block group (for a total of at least 200 households) was defined as a NORC by Lanspery and Callahan ^[28]. Sixty-five was chosen as the demarcation line instead of 60, based on Hunter’s recommendation, because it provides an estimate of NORCs that is more conservative, as 65 is the Medicare eligibility age. Lanspery and Callahan ^[28] set a minimum number, i.e., 200, rather than household proportions. They were concerned with the opportunities that NORCs provide for supportive services. The 200-household threshold represents the mid-range of the scale generally considered sufficient to support a full-time services coordinator in senior housing. In New York, a NORC is defined as a region where at least 50% of households have a senior citizen or a housing complex with more than 2500 elderly residents ^[29]. The Atlanta consortium of local providers targeted NORCs as areas where 25% or more of the population is over 65 years of age in order to provide comprehensive service delivery to the population ^[13]. The consortium further determined that a census area with a higher proportion of people aged 75 and older living alone was a high-risk area. Lyons and Magai ^[30] defined a qualified NORC as having 65% or more residents aged 50 plus but did not explain their choice of housing community. The U.S. Department of Health and Human Services issued a report that tried to identify the cut-off boundary on the age and number of older people in a NORC. They interviewed experts on ageing issues, and some people supported the use of 60 years old as the minimum for those who are considered seniors in order to be consistent with the Older Americans Act that defines the term ‘‘older individual’’ as an individual who is 60 years of age or older. However, others believed that the boundary should be determined by degree of disability rather than a specific age. The definition of “older” refers to people aged from 50 to 65 years old, while “significant proportion” is defined as 40% to 65% ^[31]. One survey by the American Association of Retired Persons in 2005 found that 36% of respondents (55 and older) lived in NORCs ^[32]. In 2006, the U.S. federal government specifically presented the definition of a NORC under Title IV of the Older Americans Act 1965 as “a community with a concentrated population of older individuals, which may include a residential building, a housing complex, an area (including a rural area) of single-family residences, or a neighbourhood composed of age-integrated housing where (i) 40% of the heads of households are older individuals; or (ii) a critical mass of older individuals exists, based on local factors that, taken in total, allow an organisation to achieve efficiencies in the provision of health and social services to older individuals living in the community, and that is not an institutional care or assisted living setting.” ^[33]. With the development of geographic information system (GIS) and big data technology, Rivera-Hernandez and Yamashita ^[21] identified NORCs in the U.S. by GIS and employed the definition of 40% or more homeowners and renters aged 65 years and older. Due to the fact that older residents may need assistance or care regardless of whether they own a home or not, the numerator was homeowners or renters. Based on the availability of ABS census data and population scale in Australia, E and Xia ^[34] introduced an Australian version of the NORC that is defined as a community with 40% or more household members aged 65 years and older. The study used the concept of household members who usually reside in private dwellings as the basic unit to define NORCs, which excludes holiday visitors and persons who have moved to nursing homes. Table 1 shows a variety of definitions of a NORC by different authors or organisations. It is important to distinguish between the two benchmarks, i.e., the proportion and number of older people for defining a NORC, because the proportion of older people helps to describe a community’s character, while the number of older people has a greater impact on the implementation of supportive service programs. In densely populated urban areas, the proportion of the population meeting the selected age criteria may fall below the selected threshold and, therefore, not meet the NORC definition. In practice, however, the number of older adults may exceed the threshold where economies of scale could be realised. The concept of NORC supportive service programs (NORC-SSP, which will be discussed in detail in the following section) is frequently mentioned by some authors and experts when defining NORCs and sometimes used interchangeably. Separating the two concepts, however, has its benefits. NORCs are communities of people, some of whom may require services; NORC supportive service programs may be a valuable addition to such communities. There may be a large percentage of older adults in NORC communities who do not require supportive services. There may also be residents in other non-NORC communities who need supportive services.