Senility

You have to get more exercise in order to stave off senility.

Source: tatoeba (326390)

The positions of eminence and authority in Congress are allotted in accordance with length of service, regardless of quality. Superficial observers have long criticized the United States for making a fetish of youth. This is unfair. Uniquely among modern organs of public and private administration, its national legislature rewards senility.

Source: tatoeba (6287156)

Inwardly the Second Men differed from the earlier species in that they had shed most of those primitive relics which had hampered the First Men more than was realized. Not only were they free of appendix, tonsils and other useless excrescences, but also their whole structure was more firmly knit into unity. Their chemical organization was such that their tissues were kept in better repair. Their teeth, though proportionately small and few, were almost completely immune from caries. Such was their glandular equipment that puberty did not begin till twenty; and not till they were fifty did they reach maturity. At about one hundred and ninety their powers began to fail, and after a few years of contemplative retirement they almost invariably died before true senility could begin.

Source: tatoeba (8855497)

Near-synonym: second childhood

Source: wiktionary