Senility: 1. Originally, old age. 2. The physical decline associated with old age. 3. The mental decline once associated with old age but now known to be due to dementia, as for instance Alzheimer's disease and atherosclerosis. "The road to senility is paved with plaques." (Paul F. Wehrle, MD [1921-2004] pediatrician and vaccine researcher who helped eradicate smallpox)
From the French senilite, from the Latin senectus meaning old age, old men, gray hair.