If you think Fantage's really fun, then do it. When you think it's getting boring, you can always cancel it. (You can do that with Fantage memberships, right?)
I got a year-long membership for another online game for my birthday last year, when I turned 14. And I'm sure you aren't the only teenager who wants a membership for a kids game. Age is just a number, you know.
..But I don't think you should announce in real life or something, you just might get teased for it.