But, as gazmog noted, that would be the boots on the ground. The Starfleet officers are more akin to experienced officers of the line, who are likely going to be older and seen more action.
Without going too far off track given what we are discussing is all made-up - go and look at the history books - the 'experienced officers of the line' at the front were often not in their 50s and 60s, they were in their 20s and early 30s - partly because war time rapidly grows the services and creates slots and bullets take care of the rest. This idea that the young were simply "boots on the ground" is a false one I think created by how TV shows are cast (and here by the fact that this production features a lot of what we in the UK might em..call 'resting' actors).
Just as a random example - Colonel John Landers who commanded the 78th fighter group was 24 and got there with four years service.
I am fully aware of this fact, but one must also look at the life expectancy rate, and when there initial military experience would have begun. Historically, in some cultures, I would not be married until the age I am now (30) and would be working in my field or the family business (I'm not).
So, while historically, if Axanar were to be painted as a WW2 piece, then yes, the officers of the line would be much younger than the current cast. However, given that life expectancy rate is longer in the Federation (its even longer now than before) I don't find it unreasonable that these seasoned officers would be their ages.
Though, given history, I probably should be less harsh on Jedi Masters being only 18 in other fan films
