The traits the word 'childish' addresses are seen so often in adults that we should abolish this age-discriminatory word when it comes to criticizing behavior associated with irresponsibility and irrational thinking.
[Kate refuses ALF to baby-sit Eric]
ALF: But why, why?
Kate: Why? Cause you're irresponsible. You trashed the living room, blew up the kitchen, wallpapered the shower...
ALF: It was a rhetorical question.