IDIOMS

second childhood

(idiomatic) A childlike state in any adult, resulting from mental illness, trauma, or other conditions.

Example: 1549, Hugh Latimer, The seconde [seventh] sermon of Maister Hughe Latimer which he preached before the Kynges Maiestie in his graces palayce at Westminster, ye xv. day of Marche [-xix daye of Apryll], M.ccccc.xlix:
  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.
1846, Charles Dickens, Dombey and Son, ch. 52:
  "You think I'm in my second childhood, I know!" croaked the old woman.
1951, "Second Prime," Time, 16 Jul.:
  Old age is not always second childhood, says he. "There is often, instead, a second prime."
1838, Edgar Allan Poe, The Narrative of Arthur Gordon Pym, ch. 11:
  Although at the commencement of the voyage I had been in bad health, and was at all times of a delicate constitution, I suffered less than any of us, being much less reduced in frame, and retaining my powers of mind in a surprising degree, while the rest were completely prostrated in intellect, and seemed to be brought to a species of second childhood, generally simpering in their expressions, with idiotic smiles, and uttering the most absurd platitudes.
1891, James M. Barrie, The Little Minister, ch. 13:
  But take care of yourself; a man's second childhood begins when a woman gets hold of him.