My question is, although aptitude was of course important, why were Bomber crews averaging out at age 21, surely many older men would have volunteered and that average age be somewhat beyond that mark.
My father had his first flight in a DH-82 (Exercises 1 & 2) in March 1937 at the advanced age of 24, and ended his RAF career in October 1943 at the age of 31 when he was shot down near Munich in his Lancaster III as a Squadron Leader.
While flying and later in Stalag Luft 3 he was usually addressed as Grandpa, and he told me he often felt like one among the 18 - 21 year-olds he was with, while he was married with 2 children.