There is a Thor in the avengers team and he is older than Captain America. Why do we call Captain America the first avenger?
The Avengers is an Initiative: a sanctioned team brought together to defend the world against threats beyond the scope of SHIELD and the world's governments.
Captain America is the first Avenger not only because he was the first to be recruited (Thor doesn't even know about the Avengers until partway through the movie: even Stark was aware of their existence before then), but because the super-soldier program of the 1940's was a wartime pre-cursor to the Avengers program, albeit one that was stalled by espionage.
Captain America was quite clearly the first member of the team to sign up to a government initiative to defend innocent life.
Ruben Studdard was 25 when he won American Idol in the 2nd Series, and Kelly Clarkson was only 20 when she won the first series. We don't refer to Studdard as the first American Idol winner, do we?