I think there might also be another selection bias here, in that StackOverflow might be a bit self-selecting for developers who know their stuff.

It's hard to say what this says about developers as a whole, including the ones not on SO, which I assume is a large number.

An alternative hypothesis might be this: Good developers get better over time, whatever that means.

I would suspect that people who form bad habits early on don't enjoy the same benefits of experience as those who built on solid foundations.