Abstract Background The aim of this study was to examine the trends in impairment regarding activities of daily living (ADL), physical performance, and cognitive function among the oldest-old (those aged 80 and older) in China between 1998 and 2014. Methods We used data on 34,297 oldest-old individuals from the seven waves of the Chinese Longitudinal Healthy Longevity Study. We estimated age, period, and cohort effects on the prevalence of self-reported ADL impairment, tested physical performance and cognitive function impairment using the age–period–cohort model. Results Regarding age, the prevalence of ADL, physical performance, and cognitive function impairment were highest in the centenarians, but they did not increase with age in this population. Among the literate subgroup, the prevalence of cognitive impairment increased more rapidly with age than that in the illiterate subgroup. Regarding period, the prevalence of self-reported and tested physical impairment slowly increased between 1998 and 2014, but cognitive impairment remained stable. Regarding cohort, ADL impairment continuously decreased. However, physical and cognitive impairment remained stable after a brief decline in the early birth cohorts. Conclusions The results suggest that the age effect is still the most obvious effect regarding several types of functional impairment. The likelihood of a younger person experiencing functional impairment may not change significantly, but ADL is likely to be amenable to improvement resulting from improved medical and social care. Therefore, increased care for the oldest-old may considerably improve their quality of life, particularly regarding their basic ADL.