Sometimes people just fail to develop into social or intellectual maturity. Maybe it's due to a very loving but confining mother or father. Or maybe they just didn't want to leave the nest. Maybe they've been intentionally secluded from learning about the world. Maybe it's the result of brain damage or something more sinister. Perhaps they just never had a life-changing moment involving a shotgun and a beloved pet. Maybe it's just a form of Mars and Venus Gender Contrast. Who can say? One of the Kids is related, in where their childishness is caused by spending a lot of time around children.