Im looking for a bike for my little brother, he is 12 years old and weighs about 75 pounds tops. But he is very athletic. I would say he is a beginner, just rode around on trails with me before. He has rode a couple bikes but the most solid one he has gotten used to was an 06 CRF100F. it was a good size for him. only problem is i sold it since it was my bike to get another bike. now he wants to ride and i wish i kept it. i just dont have the money to buy another crf100. i would like to know what other bike sizes are similar maybe a ttr 125? crf80? even an older xr100 would do. and i would say look for a 4 stroke more then 2 stroke for him as a beginner.