Twitter has always been more of a 'broadcast to followers' than a conversational tool.
Can you substantiate this in any way? This doesn't match my experiences using twitter at all.
Given that teenagers are in general the most narcissistic group of all humans - seemingly by biological specialization - that they should be the biggest tweeters.
I believe the reason teens aren't bigger twitterers is because they're not being exposed to it. Online membership is a rather passive recruiting method, it's not like boy scouts where there's someone coming into your classroom trying to get you to join. Teens will have to find twitter largely on their own, which if Twitter isn't advertising to teens it means it largely won't be found until it goes viral in schools like MySpace did in its day, notably another very narcissistic site.
Teens in more affluent areas, with better phones and messaging plans are likely more twitter users simply by virtue of better access.
Perhaps not coincidentally, I don't understand why anybody would use Twitter. I mean, I certainly understand why you'd post stuff to Twitter. I just can't imagine why anybody would ever read anything that had been posted there.
As far as I can tell, it's a huge population of people shouting to promote themselves. If there were actually anybody listening, that might be useful. But I don't think anybody actually is. Or at least I can't imagine a personality type who would do so.
1) Interesting links/insight, if I see someone posting good stuff on HN I might follow them on twitter.
2) Keeping track of events in the tech/startup sector, there's nothing that even comes close to twitter in terms of allowing you to keep an eye on what's going on
3) Staying in touch with people you meet at conferences, etc.
If you're a business then Twitter gives you an invaluable source of feedback, people might not complain to you when they get frustrated by your product but they may well complain on their twitter. It's also a great source for customer leads.
It's great for getting news from people you find interesting (let it be Paul Graham, Tim O'Reilly, a creator of language you like). Also, many projects announce new releases on twitter (ruby's twitter gem for instance http://twitter.com/#!/gem). Best of all: you know all these updates are short.
Besides you can favorite/RT items you think are interesting for later retrieval (using delicious to save twits is an alternative, but a cumbersome one).
On the sending side, it is good if you are one of such interesting people, and whish to keep people updated. Setting up forums/google groups just to give notice of a new release is too much of a overhead when you have twitter.
These are the information retrieval and useful aspects of Twitter, and are useful even if you have no friends/colleagues.
There is also the social one, which you people make of it as they want, like a sms broacast for setting up dinners with friends. If you have enough followers, you can use it effectively to ask for information. You can use it to ask people short questions, and engage in quick conversations with people that would not otherwise answer an email.
There are many other uses. In a nutshell: twitter is what you make of it.