I read somewhere about how in Joss' original script The Master was supposed to be 600 years old, thereby making him only 200 when he sired Darla. But yet he still had his demonic appearance. So at some point Joss just decided he should forget the specific age of the Master.
I've watched my Buffy (and Angel) DVDs many times over the years, and can tell you with certainty that the specific age of The Master was never mentioned on the air in either of those series....just that he's really old, according to the Council records.
It can still be assumed that he's much more advanced in age than old Drac, since the Dark Master....bater is only about 600, give or take, as has already been mentioned by others here.