Honestly it's not most as many speculate its just the individual groups varies. Remember that majority of the current day street gangs didnt blew up overnight. It took decades of absorption, partnerships, split ups , and reputation to get where they are now. In the beginning you had to be tough to join and make it through the 50-60s in L.A & Chicago. Chicago will always have the history of organized gangs and back then it was a lot of meetings and arrangements going on and still today. Lets just say that the rabbit hole goes deeper than what people on the outside knows. I digress.

Gangs do have selective membership , you must be loyal, tough, and/or hustler. They recruit kids because its easier to mentor into the group, law penalties, and of course some are born into it. But not all recruit kids and others recruit young adults. Juveniles only make up 40% of America's gang population. The rest are adults.