JS,
I bought the ttr for my son, who is seven, and had been riding around on a pw50 for a few months. He was too big, but not ready for a clutch, and I think the ttr is a perfect transition.
So far, my son and newly interested 10 year old daughter have just been riding around in second gear. More than enough torque and top end that way. Getting used to the foot brake keeps them busy enough, so shifting will come next. The motor is very flexible, with no topend hit at all. It just keeps pulling along. And the meager suspension seems to work great with their 55-75 pounds. A bit overwhelmed by my 160, but still fun.
The only problem so far was an improperly routed throttle cable, which ended up getting stuck at half throttle. Fortunately, it happened before my son put it in gear, so no harm. I was at a track that day and checked a bunch of ttr's, and half of them were routed wrong (all the new ones). Pretty obvious and the shop and I should've seen it. Yamaha replaced the cable no charge.
If your son doesn't like, you'll find that it's a great bike for putting around the neighborhood:D
Mark