In the Hurt study, riders who grew up riding dirt were statistically under-represented in crash statistics.
I'd say to get them into an ERC and have them read some instructional books before taking them out on the road with you. If you feel they're vigilant enough, and skilled enough, turn them loose with their streetbikes. When they turn 16 or 18 or whatever, they're going to go off on their own and do their own thing anyway. That's what young ADULTS are supposed to do.
Teach them to wear the gear, teach them to keep aware and keep their head in the ride whenever they're on a moto, and that if they want to ride like a goon (over 75% capability), do it on a closed course. The public roads are no place for 80%+ goon riding if you aren't ready to kill yourself or somebody else.
'8yrs in the Marines, 'almost a decade of knocking heads in the service industry, I took a ride out to visit family and friends for Thanksgiving at 41yrs old, and my mom confiscated my bike.