Requirements vary from place to place. The best way to find out what you need to know is to contact a local massage school. I can tell you that many of them do require you to be 18, and being in this field, I would not recommend this career - or even beginning the training for it, as there is a lot of hands-on experience required - for anyone younger than that. You'll be getting into people's personal space and handling their bodies, and it takes a person of a certain maturity that most teenagers simply do not possess yet.
Usually there's a little bit more required of you than simply showing up to class for a certain number of hours in order to be able to get your license. It's not just about attendance; you have to make the grades and graduate from the school (which requires a certain number of practical hours outside your class time), and in most states, you then have to pass a national exam. Not to mention all the money this requires, for tuition, supplies, exam application fees, licensing application fees... And you have to keep your credentials current once you have them.
It also usually takes more than just a few months. My program was 500 hours and it ran for an entire year. Since students in massage school are adults and most of them have jobs and families and other commitments to juggle around school, most programs are geared toward being accomodating to them. I think it would be difficult to find one that met eight hours a day, five days a week like high school. I could be wrong, though.
You can't just get licensed once and come back to it five or ten years later; there's continuing education to keep up with and renewing your license every few years, and in most states you have to have documentation that you have done a certain minimum number of hours of massage in the interim before they will allow you to renew your license. (Actually, that might be the NCBTMB that says that; I just know I have to do it.) So in my opinion, it's not worth it just to do it because.
If you are interested in anatomy, kinesiology, learning about options to compliment conventional healthcare, it's great. If you have a compassionate heart and love to help people improve their morale and sense of well-being, it might be a good choice for you. On the other hand, if you aren't comfortable touching strangers and invading their personal space (you will be working on glutes and adductors and other semi-personal parts of the anatomy), or if you have an aversion to certain body parts like feet (all feet, not just the freshly clean ones), then reconsider.