With the success of the Army's Super Soldier Project, a.k.a. Captain America, every branch wanted one of their own. Popeye was the Navy's attempt at a Super Soldier.
He is stronger than any known man, including Captain America, and single-handedly sank an enemy Destroyer, with his bare hands. Despite his strength and battlefield success, the Navy hid Popeye from the public eye due to what they deemed were "Abnormalities" namely his giant forearms and chin. He fought alongside Captain America on numerous occasions and even rescued him several times, but you won't see any mention of it in the papers or newsreels.
Popeye ages at the same slow rate that Captain America does however while Cap was frozen in the Arctic, Popeye lived his life, hence his more aged visage.
