Shaw Taylor claimed that the age of the "the latest Master" was 700 years. (TV: Police 5: The Master)
By the time of his death at the hands of and subsequent regeneration into Missy, the Saxon Master claimed to have lived for millennia. (AUDIO: Masterful)
Behind the scenes[]
According to The Doctor Who Role Playing Game by FASA, the Master was over 700 years old by the end of his fifth incarnation through his sixth and seventh incarnations, over 800 years through his eighth to twelfth and thirteenth incarnations, and over 900 years old in his fourteenth and fifteenth incarnations.