There are many fine places to dive all up and down the coast. Facing west as Santa Barbara does, you may see more rough conditions than we do in Los Angeles.
But first of all are you too young? Yes and no.
No because the dive training agencies will train and and at 15 will certify you with parental concent and a medical clearance.
Yes, in my opinion you are too young to dive.
Diving is all of the following:
1. A great Sport
2. Good for physically fitness
3. A lot of fun
4. Exposes you to increased pressures which may not be good for growing bones etc.
5. A SPORT THAT CAN AND DOES PARALYZE FOR LIFE
6. A SPORT THAT CAN AND DOES KILL even experienced divers.
Not trying to scare you with the above, but there are many facets to diving that even I at 48 yrs old, did not fully comprehend when I got certified.
SCUBA diving is definitely a high risk Adventure sport. One moment of forgetting what to do, or panic could result in death or paralysis for you or your dive buddy. Yes, your actions underwater can impact other divers.
A simple act like forgetting, and holding your breath under water while ascending even a few feet could result in a blown lung and possibly even death. Even if you follow all the rules, people still get killed. We dont know why, but even when all rules are followed some times divers get the Bends (Decompression Sickness) or an embolism (big bubble in brain) which can result in any or all of the following: 1. pain (mild to I wish I was dead severe), nerve damage (for life), paralysis, or even death.
Diving requires maturity, the ability to think for yourself under pressure and to calmly consider all options before acting in a life and death emergency situation. Do emergencies really happen? Yes, you are dealing with equipment in a very harsh environment and it can and does malfunction, plus the human factor (Murphy) and the unpredictability of the ocean. At the age of 20, some people I know are not ready to be certified as divers.
More information
http://www.scubabyte.com/scubadivingsafety.htm
http://www.scubadiving.com/training/medicine/age&dive.shtml
http://www.scubadiving.com/training/lessons/sept01.shtml
I hope that helps. Read up on SCUBA and then decide. Waiting is always an option and often the sign of maturity. In general the best piece of safety equipment a diver has is maturity and clear thinking in any situation.