The only bad thing about a CRF150 "F" is it is heavy. I think 20-25lbs heavier than the TTR. It is a better constructed bike and you can see it in the components. I am actually selling my sons's CRF150F and replacing it with a CRF150R Expert. I will put a Rekluse in it for him. Gone will be the e-start but it is nearly 50lbs lighter than the 150F. I bought two of these 150F's late last fall and sold one of them already. They both had headlights, not real bright but doable at night to get off the trail.
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Its hard to say about the YZ80 or 85. I grew up riding them as a 12 yr old kid. In fact I had a YZ80 and a RM80. Liked my YZ better at the time. It all depends on where you ride as to what I would get them. My 12 yr old and I like to do single and double track rides. Sand is hard for kids because of the smaller tires.
Harrison rode the Slick Rock Trail, Dead Cow Tube and the Tubes with the big boys a week ago. The sand killed him and really slowed him down. There were a few places that I rode his bike for him over or up an obstacle but all in all, he handled it. I never push him past his comfort level but at the same time I MAKE him ride the spots I know he can handle. It doesnt get any better than when your son or daughter can ride with you. My older son or daughter would have ridden the trails just fine.