Enjoyment from infant-related things. Pretending to be a baby, wearing diapers (see diaper lover, speaking like a child (ie "I sowwy"), sleeping in a crib. Wide range of things, but all infant-related
Note that this is interest in the trappings/behaviour of youth, not sexual interest in the youth themselves (that's a pedophile)
Alex: Today, in class, my friend and I kept laughing then annoyed the teacher. I must do it again soon!
Deeanna: Oh Alex, you're so infantile.
Alex: I know, I can't help it.
Person1: You're a jerk.
Person2: Butthead.
Person1: Infantile Pillock.
Person2: ... did you just call me a small dick? YOU'RE the infantile pillock.
Person1: Haha.
Person2: What's funny is that you aren't disagreeing...