Several early-20s characters are played by actors in their early-to-mid 30s (e.g., Bull Randleman, Blithe, Buck Compton, John Martin, Guarnere). Col. Sink, in his late 30s, is played by Dale Dye in his late 50s. Most of the actors are older than their historical counterparts, many of whom turned 20 in 1942.
Inverted for Liebgott. Historically, he is supposed to be older than most of the others in Easy Company (he was 32 by the time he was discharged at the end of the war), but he's played by a younger actor.
The Dawson Casting even induces a bit of a Tear Jerker because some of the actors were older than the characters they played were when they died. It's also worth pointing out that war does age a person rather strongly.