First, Let's distinguish between master diver and divemaster. Master diver is just a recognition given to those who have logged 50 or more dives and have taken 5 specialty classes, one of which is stress and rescue. A dive master is someone who can lead certified divers on underwater tours. SSI's divemaster certification is part of the Dive control specialist (divecon) program which is a divemaster and assistant instructor in one. The divecon rating stipulates you must be 18 or older, and there for you must be 18 to be a divemaster.
as far as an age for master diver, you can be a junior diver (under 15) and take all the required specialties (except for deep diving, you cant be a junior diver and be trained to deep dive). We have a girl in our shop who was a master diver before she was 15, so to answer your initial question, yes you can be a master diver before the age of 15.
Hope this helps
