I would let my son but he would build up to it. I'm sure this boy in the video did as well.
Last summer we saw a 13 yr old on a 250 at the MX track. He had to use a foot and a half tall block to get on the bike. The bike looked huge under the kid. The kid flew on the bike due to his weight though. He was something to see.
I can see my 10 year old wanting to do that some day.