From Longman Dictionary of Contemporary Englishsecond childhoodˌsecond ˈchildhood noun [uncountable] MENTALLY ILLsomeone who is in their second childhood is old, and their mental abilities are greatly reduced
Examples from the Corpus
second childhood• Peter, a 55-year-old distribution engineer at the plant, says his wife believes he is going through a second childhood.• Lilian was in her second childhood and Harriet was expecting her second child, which she secretly despised. 