Usually it takes until around age 15 or so. Puberty and discovering sex drive is number 1, and unhappily drugs or booze. They will indeed be shunned, given no support for maturing.
In spite of being bible based(sarcasm)the local boe may encourage kicking them into street or pawning them off on non jw relatives.
I have known a lot. A few smart alec snots make it all the way to becoming superfine bigwigs pushing their own way without getting caught