second childhood

noun2

1. •
   The period or state of cognitive decline of an elderly person, characterized by childlike judgment and behavior.

   • King David being in his childhood, an old man, in his second childhood, for all old men are twice children, as the proverb is, Senex bis puer. — 1549
2. •
   A childlike state in any adult, resulting from mental illness, trauma, or other conditions.

   • But take care of yourself; a man's second childhood begins when a woman gets hold of him. — 1891