Since no one else has said anything I will have a go.
Now I understand how you must feel if say a 17 yr old raced as an under 15 or a 19 as an under 17. I know when I was that age I could easily have passed just on appearance alone.
However, the problem I see is with implementation. Most people dont carry ID to show how young they are
, it is usually to show that they are over 18. The most likely form of ID that they would have would be their birth certificate and maybe passport, but who carries that with them. It wouldnt surprise me if when applying for an MTBA license it wasnt checked either.
The best way to do this would be to confirm their age with their parent/guardian, but if they are in on it, what can you do?