You are definitely not too old to start. However your success will come down to your dedication and work ethic. I highly recommend joining a martial arts school. If you can't find a MMA gym, then look for a BJJ or Muay Thai school. Also, and I can not stress this enough, join your high school's wrestling program. This will test your dedication and teach you a solid work ethic. If you don't like wrestling, you probably won't like MMA.
I'm not a big fan of DVD tutorials in place of instructor led training. I only recommend DVDs in a 'anything is better than nothing' scenario. It is more difficult to unlearn bad behavior than it is to learn good behavior.
Also, make sure you get into solid conditioning. Plyometrics and cardio, at 15 years old you are still growing. You could do light lifting, focusing on high repetitions and improving muscle endurance. Lifting heavy at your age can cause serious joint problems later in life. Getting into these habits at a young age is priceless.