Old age

Old age was the latter stage of a lifespan prior to death, and could be refered to as a cause of it. The Eleventh Doctor, having expended all twelve of his regenerations, was left at the brink of "dying of old age" as the Siege of Trenzalore reached its conclusion. He was, however, enabled to regenerate once more after being provided with additional regeneration energy "love from Gallifrey". (TV: The Time of the Doctor)