a person of the opposite sex who is noticeably younger than yourself, often caughtchecking you out. Usually they appear to be twelve years old, hence the term twelvie.
That twelvie over there was totally checking you out. You're old enough to be his aunt or something.
Mainly used to describe young (12-15) rich white boys and girls who use their parents money to buy marijuana. Then they proceed to post about it all over social media, eventually proceeding to them being caught by their rich parents. Then the cycle starts all over again.
"Did you see Lucas new post on Facebook?"
"Yeah, his parents found it."
"What a fucking twelvie!"