It is all based on how comfortable and confident you are as a rider. One answer says wait till your 16 to get a cr 125, i know plenty of kids your age (even younger) who run around on 250 and 450's. If you just stay on the trails and stuff stay with a four stroke. I am personally a fan Of Honda machines, they just seem to have less engine and break down issues. I own 3 honda and ride in Moab Utah where I live. It is hot all the time desert riding and I see more yamaha break down and trouble starting than i don any other machines out there. So if I were you just stay away from Yamaha. Stick to a 125 or honda makes that 150F. Could be a little big for you, but you'll grow into it and be able to ride it for longer without having to get a new bike for awhile.