Youth Engine sizes:
“THE MINIMUM AGE LIMIT FOR TRIALS ONLY IS 6 YEARS OLD.
6 to 9 year olds Maximum 80cc
10 to 12 year olds Maximum 125cc
13 to 15 year olds Maximum 250cc
Riders may in exceptional circumstances ride larger capacity machines, applications in writing to the AMCA office will be considered, with supporting evidence from Club Officials stating their case.”
“Motorcycle helmets, which are correctly fitted, must be worn by all officials, riders and passengers
when riding a machine, unless specified otherwise in the Supplementary Regulations.”
So 13 – 15 years old youths are okay on a 250 & adult riders can choose to wear a flat cap if the club allows.
Both of above from AMCA rule book and confirmed over the phone. see AMCA