| ① Health-centric studies | Chen and Zheng (2025), Liu et al. (2022a, b), Huang et al. (2025a, b), Fu et al. (2021), Huang et al. (2025a, b), Kumar et al. (2016) | Focused on biomedical or physical health indicators (frailty, sarcopenia, cognitive function, disability, morbidity) |
| ② Urban-biased or mixed-sample studies | Chen et al. (2021a, b), Lin and Xu (2025), Wang and Zhao (2024), Feng et al. (2015), Huang et al. (2025a, b) | Used mixed or peri-urban samples, some drew from national surveys without rural disaggregation |
| ③ Thematic non-index studies | Liu (2014a, b), Yang and Du (2021), Qu et al. (2023), Meng et al. (2025) | Addressed social, cultural and familial aspects of aging (e.g. migration, gender, leisure, or education) |
| ④ Governance or institutional models | Dai et al. (2025), Luo et al. (2025), Dou et al. (2025), Feng et al. (2020), Tang et al. (2020) | Investigated community services, canteen programs, mutual aid, or administrative mechanisms |
| ⑤ Potentially index-based or multi-dimensional frameworks | Guo et al. (2024), Sia et al. (2021), Chen et al. (2021a, b), Liang et al. (2025) | Incorporated multiple domains (health, participation, satisfaction) but without unified AAI-based scoring or rural specificity |
| ⑥ Other thematic or spatial studies | Qiu et al. (2024), Chen and Zheng (2025), Li et al. (2018a, b) | Emphasized environment, housing, or spatial inequality |