My younger son (just turned 12) has outgrown his 20" columbia mtb that has served him well cruising the neighborhood and light off-road duty. He has held off getting a 24" wheel bike and is now big enough for a regular 26" wheel with a 13" frame.
I have an extra Trek 930 mtb from around 1997-1998 that is too big for him (18" frame), but it has pretty good components (Shimano stx). He can actually ride it ok with the seat down, but it is way too tall and not safe (he has to stand on his toes to straddle the bar). Is it worth buying a 13" mtb frame and switching all the components over from the trek? We went looking at new bikes today and I really struggle to spend $350 - $400 on a qulity bike that he will outgrow again in a couple years and the components don't look any better than the 10 year old stuff on the trek.
It looks like a 13" aluminum mtb frame runs about $150 at performance bike, but I'm not sure if the standards have changed for head tubes, bottom brakets, etc. to allow me to pull all the stuff off the trek. I'm pretty mechanical and enjoy projects (especially with my son), so that part doesn't bother me.
Does swapping the parts to a small frame make sense or should I just go get him a new bike? What are the likely gotchas and what can I expect to buy (besides the frame)?