Aging in disguise: age, period and cohort effects in mobility and edentulousness over three decades

By corroborating cross-sectional with longitudinal analyses, this study illustrates how cohort effects can confound trends over age and time. Mobility (walking difficulties) and edentulousness (toothlessness) were studied from 1968 to 2002 in a nationally representative panel aged 18-75 (5 waves, n approximate to 5,000) and ages 77+ at later waves (2 waves, n approximate to 500). Three analyses were done: cross-sectional 10-year age group differences in 5 waves, time-lag differences between waves (shifts across time) for age groups, and within-cohort differences between waves for 10-year birth cohorts followed over time. Complementary age-period-cohort models using logistic regression analysis evaluated differences. Both mobility and edentulousness have earlier been shown to be strongly related to age cross-sectionally. For mobility, cross-sectional and longitudinal analyses showed large changes, whereas time-lag analysis indicated no or marginal changes. Both cross-sectional and longitudinal results showed an exponential curvilinear age dependency for mobility limitations, with limitations becoming more usual in older ages. In contrast, cross-sectional and time-lag analyses of edentulousness showed large differences, whereas longitudinal analysis indicated no or marginal changes. Rates of edentulousness became increasingly lower for successively later cohorts in a curvilinear fashion. These patterns demonstrate that age effects dominated mobility, whereas cohort effects dominated edentulousness. Age-period-cohort models confirmed these findings. The cohort effect of edentulousness implies that the cohorts' movement through time gives a false impression of age and period effects in cross-sectional data.