I would like Chris Pine as Captain America...I liked him when he was in the running for Green Lantern as well. I agree with the muscle. Wasn't Cap like 19 when he volunteered for Operation Rebirth? He's probably operated in the modern era for ten or fifteen years by now. The movie seems to be divided into two halves, the majority taking place in World War II which in case Captain America should be in his early twenties (given a few weeks for boot camp training) and then the same for the end of the movie since he's frozen in ice for sixty odd years and found by either Howard Stark or Nick Fury (since we see a glimpse of the adamantium sheild in Iron Man we can surmise that they find Cap's sheild first).
Steve would seem to have been in his late teens or early twenties. Not sure how anyone would think he was in his thirties in the early 1940s.