Regeniration

Time Lords are extremely long-lived, routinely counting their ages in terms of centuries; the Second Doctor claimed in The War Games[10] that Time Lords could live "practically forever, barring accidents." The series has suggested that Time Lords have a different concept of ageing from humans. In Pyramids of Mars,[50] the Fourth Doctor considers an age of 750 years to be "middle-aged". In "The Stolen Earth",[51] the Tenth Doctor refers to when his original incarnation was a "kid" at 90 years old. However, within a specific incarnation, a Time Lord is able to age, albeit much more slowly than a human. The War Doctor[34][35] and Eleventh Doctor,[52] over the course of the Last Great Time War and the Battle of Trenzalore, respectively, are seen to age within their respective incarnations to what would appear to a human to be old age; both conflicts are suggested within the series to last hundreds if not thousands of Earth years.[note 1]