Aside from the obvious ambiguities of what defines sophistication of a culture: It feels like you're begging the question to me. "Americans are unsophisticated because they're a young culture so they're unsophisticated..."
Moreover I think suggesting that an entire culture is full of optimism and youth because it's only 300 some-odd years old strikes me quite sweeping. The modern nation state is barely as old, and it's not as if Americans don't have some cultural heritage that goes back much further than that.