Steve Rogers is said to have been born in the 1920's. WWII started roughly in 1939 with the invasion of Poland by Germany. Rogers attempts to enlist in the US Army in 1940, however due to his frail physical state he is denied entry.
Having been observed by certain key Army officials attempting to enlist several more times after being denied, he is inducted in to Project Rebirth as its first candidate for Dr. Erskin's Super-Soldier Serum. After receiving said serum, and becoming the pinnacle of human potential he operates in the European theater of war for several years alongside Bucky Barnes and, on occasion members of the Invaders.
On his final mission during the war he and Barnes are presumed dead when the experimental bomb they are attempting to stop from destroying Washington DC explodes dumping Cap in the Arctic waters to be frozen and preserved for many years before he is found and revived by the founding members of the Avengers.
I'm trying to figure out how old he was when he received his serum treatments and became Captain America, and/or how old he was when he was frozen in the ice.