1. Vanke School of Public Health, Tsinghua University, Beijing, China 100084
2. School of Medicine, Tsinghua University, Beijing, China 100084
3. Center for the Study of Aging and Human Development, School of Medicine, Duke University, Durham, North Carolina 27710, United States
4. Center for Healthy Aging and Development Studies, National School of Development, Peking University, Beijing, China 100091