Teens don't twitter because they are in constant contact with other teens, especially through texting.
More to the point, in my actual life I know no one who Twitters (or tweets, or whatever). But I do know tons of people who have tried and given up on it.
So is it possible that, as a percentage of society, almost none of us Twitter?
We still have almost no evidence as far as I can tell that Twitter is being adopted as a widespread general communication tool. That's not to say that it won't, just that what we see now is still 90% hype driven and it's anybody's guess whether the thing will collapse or not when the hype dies down. For communication among my friends I still vastly prefer instant messaging which has so many features that twitter and SMS don't that using twitter feels like returning to the stone age.
Twitter is miniature compared to many of the huge networks out there. Bebo, Hi5, Orkut are all huger than Twitter, last I checked. (EDIT: I checked after posting this and Twitter's past all of them globally according to Alexa, but not in terms of users.) I don't go out of my way to analyze Facebook, but I know that a Facebook fan page having a million followers is fairly routine, even for relatively minor fan pages. Getting followers on Facebook is much easier than it is on Twitter, and it gives you the exact same relationship with your audience, but even stronger because there's comment/photo/video support.
I don't mind Twitter at all - I wish more web sites would be so elegant - but a lot of the hype about Twitter being the "next big thing" is only half true. It's the NBT in that it succinctly represents a lot of the coolest things about the Internet, but it's not the only site to do that and it's one of the smaller examples out there.