The phrase "at an age" means "at approximately the same age." It is frequently used when comparing two individuals, when one of them has done something remarkable due to either their youth or advanced age.
Examples:
At an age when most boys are playing with sticks and balls both Beethoven and Mozart were accomplished composers.
(This doesn't mean Mozart and Beethoven lived at the same time. Note also the interesting use of the present tense - boys are playing - even though Mozart and Beethoven have been dead for centuries.)
Exercise guru Jack LaLanne performed amazing feats of strength at an age when most senior citizens have trouble climbing stairs.
The phrase is particularly useful because it suggests an approximate range of ages. When I wrote my original post, I didn't know exactly how old George H.W. Bush was when he was shot down in WWII. But I did know he left University to join the Navy. I didn't want to use the more precise "at the same age" - because it is possible that he might have been been technically slightly older or younger than I was when I was a "feckless college kid."
