I am looking for a word that encapsulates "teenager-ish" - that sort of petulant, contrarian, immature behavior. Any ideas?
Bratty covers the type of traits you're talking about, although it's often used of younger children.
adolescently if you are looking for an adverb
In a adolescent manner; like a teenager.
adolescent itself is an adjective also, meaning:
Characteristic of adolescence; immature: an adolescent sense of humor.
I focused on the word teenager though, because it is different than being child. So teenagerish would be different than childish. Though they are both immature.