It's a bit regional, I think.
In NJ, the only other bike commuters I see regularly are somewhat older than me (and I'm 47). I occasionally see younger guys, but they are seasonal; I see the old guys year-round.
In NYC I see a more younger guys, and women too, and more who commute in all seasons.
Yep. The dense cities have more youngun's because cars can be so expensive and inconvenient, and there are alternatives to cars and bikes when you need them. The burbs have a different breed. Thank god for the old guys, some of them I can catch.