Steve's birthday would have been earlier -- the first issue of Captain America hit the stands in late 1940, it's just the cover date that was March 1941, and the Rebirth project took some months, so in Brubaker's version his 20th in the lab would have been 1940. Or 18th, if you go with Marvel's official timeline.
I personally go with 'Steve tried to sign up as soon as he turned 18 in 1940, got pulled into the Project, became Captain America a little before the end of 1940.' It fits both the canon timeline and fits in best with his story and desire to do justice. Since plenty of horrible stuff was happening in Europe before 1940, it doesn't make a whole lot of sense for a 19 year old Steve to hang around and then in mid 1940 develop a desperate passion to join the Army. Makes a lot more sense for him to have to wait to get to legal age and then immediately sign up.
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I personally go with 'Steve tried to sign up as soon as he turned 18 in 1940, got pulled into the Project, became Captain America a little before the end of 1940.' It fits both the canon timeline and fits in best with his story and desire to do justice. Since plenty of horrible stuff was happening in Europe before 1940, it doesn't make a whole lot of sense for a 19 year old Steve to hang around and then in mid 1940 develop a desperate passion to join the Army. Makes a lot more sense for him to have to wait to get to legal age and then immediately sign up.