Remember the Commander of the Whitestar Sheridan sent on the suicide mission to draw the Shadows to Corianna 6? He looked in his mid 30's. This fits with the timing, the Rangers had to replace many people after the Shadow war. I would think that if the Commander was more experienced and decorated, his withdrawing from battle would be looked upon lighter. Being a newer Commander, his case was treated harshly, as an example to other young officers.
Also, I believe Ranger Trining Instructors seek the skills of a trainee, I would imagion that if one is a born Commander, he would be trained as a Commander. A great Gunner would always be a Gunner, a great Pilot would remain a Pilot. A born Recruiter would always recruit, (we saw an episode where a Minbari was made a Recruiter instead of a Fighter Pilot,).
I think his age is moot, its his skills, abilities, instincts, and reaction to the Ranger Training.