Age trajectories of independence in daily living among the oldest old in China

With the rapid increase in the population of the oldest old (those aged 80 and over), there is some concern how longer life might be associated with a loss of independence in daily living. Addressing the trajectory of loss of independence for the oldest old is challenging, not only because the oldest old are heterogeneous, but also because the health trajectories at the population level may noticeably differ from those at the individual level. We used the 1998, 2000, 2002, 2005 and 2008 waves of the Chinese Longitudinal Healthy Longevity Study to assess the age trajectories of the loss of independence for three cohorts born in 1909-1918, 1899-1908 and 1893-1898, respectively, both at the individual and the population levels. Independence was measured by combining the activities of daily living and the Chinese version of the mini-mental state examination. Controlling for various confounding factors, particularly the selectivity due to death and loss to follow-up, we found that, while more recent cohorts had higher initial levels of independence, this was followed by a faster decline compared to the earlier cohorts. Also, their level of independence fell below that of their earlier born counterparts at the same ages. Decomposition analysis further illustrated that the decline of independence at the population level is more gradual than that at the individual level. This finding can be largely attributed to selective mortality and loss to follow-up. For instance, the population prevalence of independence for the 1893-1898 female cohort declined slightly, from 19.7 to 11.1% in 1998-2008, while the proportion for females who survived from 1998 to 2008 dropped from 66.7 to 11.1%. For the population as a whole, a longer life expectancy does not necessarily result in a rapid decline of independence. For individuals, longer life expectancy accompanies deterioration of independence.