We don't know. Anymore. The last onscreen regeneration age given in the Classic Television Series was when the newly-regenerated Seventh Doctor said he was 953 years old. That means the first six Doctors averaged just under 159 years each.
The novel Vampire Science says that the events of The TV Movie took place when the Doctor was 1009. That drops the average down to about 124.(The last Eighth Doctor Adventure novel was originally going to say that the Eighth Doctor regenerated at age 1370, bringing the average up to about 171.)
But then the Ninth Doctor says that he is only 900. And when we factor in the War Doctor, the average slumps down to 100. The rapid regenerations of the Ninth Doctor, the Tenth Doctor, and the Tenth Doctor made the Doctor about 906 in his thirteenth incarnation. This brings the average down to about 75/76.
And now of course we don't know how long the Eleventh(Thirteenth) Doctor lived for. He was 1200-and something before his centuries-long stay on Trenzalore. In any case, the various statements from the Ninth and Tenth Doctors stating that they are 900-something years old, as well as the War Doctor stating that he's 800-something years old totally and utterly contradict the Classic Series.