Excellent point Wines and Bobby
From memory Owl said something once (also in the Guardian or just on Twitter?) that when trans athletes have been on puberty blockers it will not be a problem (around fairness) as the natural strength etc that develop during puberty would be altered.
Only the likely damage done to bodies by late teens, early twenties, will prevent them from being athletes. If they were in tip top physical condition and at a level to compete their IQ could be affected so maybe signing a consent form as an adult with a brain that never matured as nature intended shouldn't be an option either.
(There urgently needs to be more studies on PBs.)