It takes practice and skill to get a standard bicycle going that fast

It does not. As someone who does regularly do that on rides, I can vet that any able bodied kid can get a road bicycle to hit 40 on a long downhill, first try. The real difference is that they do not go seeking that experience, usually they’re just trying to get somewhere they need to be.

Personally I’m good with “If it has a throttle-driven motor, it’s a motor vehicle” and just doing motor vehicle licenses as we always have. Maybe just lower the permit age if teens need access to motor vehicles, or ask ourselves why we built a society where teens need access to motor vehicles.