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We have absolutely no way of knowing this.

It's possible that he's around the same age as the Doctor, but by no means certain (they do always meet 'in order', but each time they meet, it could easily have been 50 years for one of them and only 5 for the other). The Doctor is around 1,000 during series 6 though.

In fact, even if we knew every second of the Master's entire history, it would still be ambiguous how old he is, given that he twice (more, if you count non-TV stories) died and was later resurrected, and we have no idea whether there's an afterlife or how time passes there.