I moved to a KTM125sx from a ttr125l when I was twelve. I moved to a KTM250f about a year later then back to an older 125, which acted as a transition bike for a few months before I got a 450, when my 250f blew up( rod bearing cage came apart; seems to be very common with these bikes). When I was 14 I got a 2008 YZ450f( keep in mind that I was 5'11" and 210lb). I had no problems adapting to the bike as I had ridden bigger bikes in the past. The reason I went with the 450f over a 250f or 125 again was to hopefully have something reliable and that I could improve on without having to worry about the engine wearing out parts, so I didn't have to spend as much ( my parents help me buying bikes, but from there I am completely responsible for them). If money wasn't an issue, I would have went to another 125 or 250f. I believe it would help you in the long run if you are trying to become a serious rider.