[pg]

I didn't have a because in there. What I said was that Americans seem unsophisticated but energetic and optimistic, and that since all those qualities are associated with youth, you could compress them into one statement by saying American culture felt young.

Nor did I say that American culture has this quality of youthfulness because the US is a comparatively young country. In fact that seems unlikely to be the cause, because there are younger countries that don't have it.

[samfoo]

I see your point, re-reading your original post. The phrase "young culture", to me, carries a heavy emphasis of literal age.

Ultimately -- and semantic quibbling aside -- I think you're right that American culture is (and by extension, typical Americans are) particularly energetic and optimistic.

I'm not sure I agree with unsophisticated, unless you mean "less formal".