puerilism

Noun

 * 1)  Childlike behavior by an adult, especially as indicating a mental disorder.
 * 2) * 1932, Aubrey Lewis, "The Experience of Time in Mental Disorder," reprinted in Inquiries in Psychiatry (Routledge, 2013), ISBN 9781134052431, p. 14 (Google preview):
 * With regard to hysteria; in dissociation there may be a secondary time disorder; for example a patient with puerilism said: "People all seem grown up and older. And my Mum, I'm sure I know her from a long time ago. I look at her, but I can't make it out."

Etymology
.