My kid was riding BMX bikes as a tyke, then graduated to a mountain bike in 6th grade. He finally got large enough in 2004 to fit onto my old road bike, a 1986 Centurion. So I let him use it.
Now he uses the road bike as much as possible. He still takes the MTB to school, because kids have been messing with bikes, and he doesn't want the road bike to be wrecked. So his MTB is now his "beater" bike.
He says that the slowness of the MTB relative to the road bike is frustrating to him. Yeah, little boys who don't really ride, but just "pose" on the bikes in front of their houses while hanging out with friends like the rugged, SUV-like knobbies... but kids who really ride to get places fast want a road bike!
BTW, my kid just did a 50 mile ride this past weekend, and got the Boy Scouting Cycling Merit Badge. Yes, I am proud!