A lot of teens are already into blogging but most are sharing personal thoughts or experiences and they do it on free blogging platforms with no thoughts of monetizing it at all. I do know of some teens, though, who made big money on blogs by reviewing gadgets on their website. It takes time to build a reputation on the web and if you can start while you're young and you actually have something worthwhile to share, then by all means invest time and money on a blog.
I think they are on the right track. Remember that many of them are still in school and are not that concerned with making money. Yet. So blogging about their personal thoughts and experiences is a good way to improve their skills. As for free blogging platforms, there are two I know of which are good. One is Wordpress and the other is Blogger. Both are established and reliable. Both are also free. It's a good place to get started. It's also a good place to keep going. One blogger I know has developed a very professional site using Blogger which she had used right from the beginning.