It's also just another effect of once face to face activities becoming internet activities. Much like how we once met dates at places called "bars" or even "classrooms," and at one point when we wanted to play a video game with someone we actually had to have them come over to our houses, tolerate their company for hours at a time, AND fork over the extra cash to buy a second controller for them to use. Now everyone does these things via the net. Well truth or dare has also made its way to being an internet activity, and some of these channels actually do encourage you to send them questions so that they can do a truth segment (sad to say that I have seen one, and the questions are even more lame and/or fucked up than the dares). My guess is that some people don't like the idea of answering personal questions on a forum where anyone can not only hear, but see their faces, so many stick with the dares. As for why it is mostly 12 year olds who do it? Well, think about it, when was the last time you got together with friends for a game of truth or dare? The reasons so many people are doing this are simple: 1. they think that it is fun. 2. it's a fad, and we all know how kids are with fads.
-BadGuy