puerilism

noun

1. (uncountable, usually)Childlike behavior by an adult, especially as indicating a mental disorder.

   “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."”

   “[T]he Dutchman Johan Huizenga, who coined the term puerilism in the 1930s . . . defined it as permanent puberty.”

   “A kind of puerilism marked many American attitudes — an unnaturally extended puerilism which tended to transmute itself into senility alarmingly and swiftly.”