An "Early Bloomer" is a super sexy and horny, non-adult teenage chick, who must therefore be avoided by horny adult males at all cost!
"God damn, look at that sexy bitch!"
"She's an Early Bloomer dude. You better stay away from that shit, unless you feel like going to prison and getting butt raped a hundred times for one fun statuatory rape."
"Thanks for the tip Bro!"
An early bloomer is a girl that has a "little girl" vagina and doesn't have a protruding clitoris or any other signs of maturity.
I can't think of any usually this is used to tease other girls such as "hey Early bloomer! this is the big girls room! go play with your dolls you little bitch!"