detinjast

1. derog. (of adults) childish; immature
   Ne budi detinjast, to je samo igra. — Don't be childish, it's just a game.
   Njegovo ponašanje na sastanku bilo je detinjasto. — His behaviour at the meeting was childish.
2. (of looks, handwriting) childlike
   Ima detinjasto lice iako mu je četrdeset godina. — He has a childlike face even though he's forty.

Do not confuse with "dečji" — "children's, for children" (dečji vrtić, dečja soba).