If we're going for an origin story, Cap needs to be pretty young. Mid-twenties to early-thirties might be around the right age for the role--young enough to play recruit Rogers and old enough to play mature Captain America.
Jake Gyllenhaal (Jarhead, Zodiac, The Day After Tomorrow) (26 years old)
Amazing young actor who's expressed interest in superhero roles before. He's 6'0, ripped, and he has a heroic air about him. I think he'd be a fantastic Captain America.
Jared Padalecki (Supernatural, House of Wax, Cry Wolf) (25 years old)
He's got the look, and he's not a bad actor, from what I've seen. Although, admittedly, I haven't seen much of his stuff. I think he could handle the role. Plus, the dude's about 6'4. So, he's got the height down, at least.
Ryan Gosling (Half Nelson, The Notebook, Fracture) (26 years old)
One of the best young actors working right now. His performance in Half Nelson was compared to De Niro and Brando. He's tall enough, has the blond All-American look, and is the best actor out there in the age range. One problem, though, is that he was born in Canada. As far as I'm concerned, though, that shouldn't be a problem.
Ryan Reynolds (The Nines, Blade Trinity, Smokin' Aces) (30 years old)
Great young actor. Tall with an insanely muscular build. He can play young and innocent was well as hardened and tough. One of the best for the role, in my opinion.
Michael Muhney (Veronica Mars, Disconnect, Angst) (31 years old)
This guy's pretty unknown. I really only know him from his work in Veronica Mars. But, he did a great job on that show, crafting a bastard of a character that had a touch of humanity to him. He could turn it around into playing a good guy, I'm sure. And, he looks right for the part.
James Van Der Beek (The Rules of Attraction, Dawson's Creek, Texas Rangers) (30 years old)
Admittedly, not my favorite actor, but he's a good one, and he has that all-American look Cap needs. And, let's be honest, Van Der Beek needs a big role, and he'd work hard to make his performance great.