Stamina also comes with age. The young sprogs can generally wing the fitness thing. The mid-20s have often lost the physical fitness edge. The 30 plus guys can often continue for longer.
Then there is the acceptance thing.
Simply the 18 yr old by the time he has been commissioned 8 years will know his stuff and he will be able to earn the respect of the men. A 26 yr old that has been in a year will be treated the same but won't have that 8 years experience to draw on.