"My son, be steadfast in honoring your father; do not grieve him as long as he lives. Even if his mind fails, be considerate of him; do not revile him because you are in your prime." (Sirach 12-13).
Dementia stands out as a signature affliction of old age throughout human history. Defined as symptoms linked to loss of memory and thinking skills, dementia takes several forms, with 60-80% caused by Alzheimer's disease. Yet, it is not considered to be a part of normal aging.
Some moderate cognition changes are expected with age, but these do not lead to loss of independence. However, more serious cognitive decline can produce symptoms like rapid forgetting, navigation impairment, inability to solve everyday problems, or difficulty holding ordinary conversations.
Nevertheless, age is strongly correlated with dementia. One estimate shows only a 1.7% prevalence of dementia among those 65-69 years old. But with a doubling of new cases every five years, dementia incidence grows to over 50% by age 90.