Career Progression
Andyy touched on career progression at the bottom end and certainly an early commision as a watchkeeper would be useful at the other end I would guess.
In the RAF a career progression might be:
IOT 24, FTS 25, Sqn 27 as a flt lt graduate. 29 2nd tour as senior flt lt. 32 sqn ldr and 37 wg cdr. Add in a couple of years for staff colleges. 43 gp capt and 47 air cdre. 51 AVM and you would then be too old to become an AM even.
Move the entry date and the upper ranks fall off the tree. Starting as an aircrew wg cdr at 40 is as late as the fast mover can afford and then would probably cap at gp capt or air cdre.
CAS joined at 21 and became an AVM at 48 and ACM at 53 so my hack is about right.
You may only wish to rise to sqn ldr but just maybe the system is keener to give all the chance of rising to the top.