I think letting a 12 year old ride a 250 is fine and most of you are way too worked up about it
Last month at my cousin's place outside Bristol, my nephew turned 12 and got a used 250 for his birthday, and half the guys in our riding group acted like my cousin handed the kid a loaded gun. The boy has been on a 90 since he was 7, he took the MSF dirt course in June, and he gear-checks that bike before every single ride without being asked. I get that everyone has a story about a bad crash, but a 250 isn't some wild machine, and honestly the 400 pound guy on a 700 who never wears a helmet worries me way more. At what point did we all decide a kid on a 250 is the problem instead of the adults who won't teach them brakes and clutch first?
A 250 at 12 is still a 250 at 12, and that bike will do 60 plus whether the kid respects it or not. My buddy's son was on a 125 at 13, hit a rut he'd ridden a hundred times, and broke his collarbone and two ribs before he could even think about the clutch. Being good on a 90 for five years does not mean your body and brain are ready for double the power and double the weight when something goes wrong.