After watching the latest Captain America instalment, part of the juggernaut that is The Avengers, it seems relevant that Cap is (by a fair few years) the oldest of all the characters. This is a grown-up superhero movie; serious, war-esque and with absolutely no gimmicks. Now, that doesn't sound like a Marvel combination does it? But with these characters, it's evident there's no other way it could be done.
