Kennedy was in the House for 6 years, and the Senate for 7. That's 13 years of political experience. Nixon was in the House for 3 and the Senate for 3. That's 6 years of non-VP experience. Both men entered politics the same year (1947). And Mr. Nixon is only 3 or 4 years older than Mr. Kennedy.
The only advantage Nixon has is that he's been Vice President; a position even Ike has said he hasn't done diddly in. So the argument that Kennedy is too young and inexperienced is bunk.
There is a very good argument that simply being "in politics" isn't enough. Looking back over the past century, the most effective Presidents (and one of the worst) were all former state governors. Experience in an executive office appears to be far more important than simply sitting in a legislature, no matter how long one serves there.
That's not to say that someone with no executive experience could be an effective President. But the odds are that anyone who was a state governor, Vice President (even if he hasn't done diddly, at least he was able to observe how the Executive branch works), or who came into politics straight from being a private sector CEO (we haven't had one yet, but there's always hope), or a flag ranked military officer.
As for who would get nominated, Stevenson couldn't beat Ike, but it's doubtful that anyone could have beaten Ike. Stevenson might have been able to beat Nixon. Humphrey had the skills, but not the power base to get the nomination. Johnson had the power base, but lacked personal charisma. Kennedy was not the best choice to run the country, but just as the winner in the election in 2008 did, Kennedy gave the best
illusion of being the right choice. As a candidate to defeat Nixon, Kennedy was the best choice.