I'm not saying this proposal will be approved. But there is a proposal going into our rules making body that basically says you must be 14 to ride a 250F and 16 to ride a 450F. If the proposal is passed in October, it wouldn't go into effect until 2017. So I would hold off on buying your 12 year old a 250F until Christmas. Call your local AMA congressman and voice your opinion.
Alex, I'm not sure how much say you have with the rule making, but if you would make sure they think it through. I completely understand the thought process with this particular proposal. I think there needs to be a means for a provisional of some manner to make exceptions. I haven't given that part much thought as I am out of the fight for now.
My personal experience with my own son was a little nerve racking at times. At age 9 he was 4' 11" tall and 105 pounds racing a 65 at Loretta's. By age 11 he was 5' 8", 140 pounds practicing on a 125 and 250F. At 13 (now) he is 6' 2" and 160 pounds. By all accounts he is not a novice rider, but as a dad, it scares the life out of me at times, putting him in with the more mentally and physically mature group of intermediate riders.
By no means am I saying the exception is the rule, but maybe there should be some thought put into a provisional of sort.
Back on topic... I believe the next stop after or during super's is deff. a 125.