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This is never revealed. The War Doctor says he was "eight hundred and something" shortly before he regenerated(in "The Day of the Doctor"). But we don't know his precise age at regeneration. The Ninth Doctor said he was 9900 in "Aliens of London", but we don't know how long he had been in this incarnation at the time.
9th Doctor

The Ninth Doctor

The Tenth Doctor said he was 903 in "oyage of the Damned", but again, we don't know his exact age at regeneration.