返老还童
In one's second childhood. An old man is twice a child.

eg：Leonard joined the football team at the age of sixty, he was then in his second childhood.

中英俚语俗语来源：神仙传：“八公曰，王薄我老，今则少矣，八公皆变为童子。”

反义词：年少老成 An old head on young shoulders.