Some Adept Doctors seem to become Masters about 60. However, I've had them as young as 16. It just depends on how much patience you have with them. I hold "Doctoring 101" every day for a bit and just pile mine on the black vine over the pool. Some go on to become masters very quickly. However, when I find my adept doctors doing another skill that has them at adept, and they have not moved out of the same place they were when they became adept doctors, I just change their skill to the one they prefer. It's odd how many doctors of my tribe like to build as well.