You're difinitely not too old but you need to jump now! As stated in the post above mine MMA is young enough still to where you can be a late bloomer and still have a chance, ten years from now when you watch the build up shows for UFC events you're going to see footage of kids rolling at the age of 4.
What you really need to do is find a good gym, and make a choice as to what weight you want to fight at. Right now, weighing 155 you should probably aim for featherweight (145). If you weighed 155 at 24 I would say aim for 135, but cutting 20 pounds at the age of 16 is very unhealthy. Start now and look at Featherweight, as you get older you will likely move into Lightweight, and then most likely Welterweight... by the time you're 25/26 you might even be looking at Middleweight. You will grow a lot over the next ten years.
Once you find a gym and a camp you are comfortable with, take as many amature fights as you possibly can. A lot of people make the mistake of winning 2 or 3 amaeture fights then jumping into pro level... you will be shocked at the competition jump in most cases. If you start training now, and start competeting at say 17, try to get at least 10 amaeture fights in... and see how you do. If you're feeling comfortable find a refferable organization to take a pro fight, and make sure you have strong representation. Then go for it.
** I do coach youth here in Saint Louis in Kickboxing... this is the most common and successful route, at least in my experience.