This obit from todays DT shows how easy it was to bluff your way in while under-age, even in WWII.
Squadron Leader Joe Blyth - Telegraph
Aged only 15, in 1940 he volunteered for aircrew duties in the RAF, having “stolen” his older sister’s national insurance number, allowing him to claim that he was 18. He was accepted and started his training as a wireless operator/air gunner in November 1940, and in December the next year he joined No 161 (Special Duties) Squadron, flying operations to drop agents and supplies into occupied Europe.