The Doctor's age

Contradictory information has pointed at different estimates of the Doctor's age, both in conversations and in terms of the length of various incarnations.

Statements and known information
It is unknown what the Doctor's life expectancy is, given his ability to regenerate. The Second Doctor once stated that as a Time Lord he could "live forever, barring accidents" though it's unclear whether this is accurate or an exaggeration. (DW: The War Games) It is also evident that Time Lords age in a different way to humans. The Fourth Doctor once said to his companion that he was a teenager for fifty years. (DWM: The Time Witch) He has also gone through spans of linear time longer than average human lifespans without growing visibly older. (EDA: Escape Velocity, BFA: Orbis, DW: The Impossible Astronaut)

First Doctor
The Doctor was eight when he entered the Academy, along with The Master. (DW: The Sound of Drums) He later described himself as having been "just a kid" when he first visited the Medusa Cascade, at the age of 90. (DW: The Stolen Earth) He said he was a "teenager" for fifty years. (DWM: The Time Witch) When the Fourth Doctor was 759, (DW: The Ribos Operation) Romana noted the Doctor had been travelling in the TARDIS for 523 years. This would have made the Doctor about 236 when he first 'borrowed' the TARDIS and left Gallifrey. (DW: The Pirate Planet)

This figure was broadly supported by the TARDIS itself. When transferred into a humanoid body, the TARDIS said the Doctor had travelled with it for 700 years. At this point, the Doctor was around 909 years old, putting his age when he stole the TARDIS about 200. (DW: The Impossible Astronaut, The Doctor's Wife) During the time Susan Foreman travelled with the Doctor, Ian Chesterton, and Barbara Wright, she knew that despite his apparent maturity, in terms of their own species the Doctor was considered an adolescent. (CC: Here There Be Monsters) He stated he had been travelling in the TARDIS for 60 years when he visited 64, putting his age at 296. (PDA: Byzantium!) The Doctor spent "centuries" studying at the Time Lord Academy. (DWM: Mortal Beloved) Magnus chided the First Doctor for not regenerating and for holding on to this incarnation as long as he did (DWMS: Flashback). This incarnation regenerated when about 450. (DW: The Tomb of the Cybermen)

Second Doctor
At some time after his first regeneration (but not long after given the presence of Jamie McCrimmon), the Doctor made the first known direct reference to his age. He told Victoria Waterfield he was "something like" 450 years old. (DW: The Tomb of the Cybermen)

Third Doctor
This incarnation of the Doctor started to say that he had been a scientist for "several thousand" years, but stopped himself before completing the sentence. (DW: The Mind of Evil) He had said something similar shortly before that incident. (DW: Doctor Who and the Silurians).

Fourth Doctor
The Fourth Doctor seemed to be 748 at his start. (DW: Planet of the Spiders) He gave this number as his age not long after, (MA: A Device of Death) and later on he consistently said he was about 757 or 749 when he travelled with Sarah Jane Smith (DW: The Brain of Morbius, The Seeds of Doom), a year older with Leela (DW: The Robots of Death) and 756 with Romana during her first incarnation just before finding the first segment of the Key to Time. Romana corrected him, saying he was 759. (DW: The Ribos Operation) Before they assembled the Key to Time, he turned 760. (DW: The Power of Kroll) Another time, he said he was possibly 730, but couldn't remember his actual age. (DWM: The Time Witch) He claimed at various times to be anything from 400 to a shade over 1000 years old. (MA: Evolution)

Fifth Doctor
The Doctor said he was 813 when he regenerated (MA: Cold Fusion)

Sixth Doctor
The Doctor said he was 900 years old while travelling with Peri (DW: Revelation of the Daleks) and later amended it to "more or less". (DW: The Mysterious Planet) At some point in his past, he had blown out the candles on his 900th birthday cake. (BFA: The One Doctor) At another time, he was nine years shy of an even thousand during his travels with Melanie Bush. (BE: A Wee Deoch an ...?) He was 953 years old when he regenerated. (DW: Time and the Rani)

Seventh Doctor
Hours after his regeneration, the Doctor unlocked a door in the Rani's laboratory with the numerical code "953," saying it was his age and the Rani's. (DW: Time and the Rani) The Doctor had his thousandth birthday while travelling with Ace and Bernice Summerfield. (NA: Set Piece) By his own estimate afterward he would have been 1009 when he regenerated, though he admitted he may have lost count at some point. (EDA: Vampire Science)

Eighth Doctor
Three years after his regeneration, the Doctor calculated his total age to be 1012. He was unsure and might have lost count, and so preferred to simply begin counting at his most recent regeneration, giving his age as "three" when asked. (EDA: Vampire Science) The Doctor aged while trapped as an amnesiac on Earth in "real time" between the late 19th century (EDA: The Ancestor Cell) and the year 2001, making him at least 1125 by the end of this crisis. (EDA: Escape Velocity) While travelling with Lucie Miller, the Doctor spent 600 years on the planet Orbis. He later mentioned that he rounded and adjusted his age based on different year lengths in different parts of the universe. (BFA: Orbis)

Ninth Doctor
This incarnation claimed "900 years of time and space", i.e. travel in his TARDIS, and said that this was his age shortly after bringing Rose Tyler back home. (DW: Aliens of London) He later claimed to have used the name "The Doctor" for nine centuries and to have had "900 years of phone-box travel" while with Rose and Captain Jack. (DW: The Empty Child).


 * See 'The "900" Controversy' below for an out-of-universe discussion of this change.

Tenth Doctor
The Doctor said he was 903 years old while attempting to rescue the Titanic. (DW: Voyage of the Damned) The Doctor's claim is questionable. Prior to the Titanic incident he is known to have spent at least 33 months searching for Martha Jones in deep space (DW: The Infinite Quest), and several months (at least) living in various time periods on Earth and inside the Valiant. (DW: Human Nature / The Family of Blood, Blink, Last of the Time Lords) He again claimed 903 during an adventure with Donna Noble. (NSA: The Nemonite Invasion) In a later adventure at in Nevada, he said 900 again, suggesting he was rounding down. (DW: Dreamland) Just prior to regenerating, he gave his age as 906, suggesting that several years had passed for the Doctor after his travels with Donna Noble. (DW: The End of Time)

Eleventh Doctor
Not long after Amy Pond first encountered the Doctor, he told her that he was 907. (DW: Flesh and Stone) and again when meeting the Dream Lord. (DW: Amy's Choice) When his future self invited him to Utah, the younger Doctor gave his age as 909. (DW: The Impossible Astronaut) The Doctor estimated that he was around George's age "about a thousand years ago". (DW: Night Terrors) When he met Amy, Rory, River and Canton at Lake Silencio, he told Amy that he was 1103. (DW: The Impossible Astronaut)

The "900" Controversy
Before the Eleventh Doctor's 200-year farewell tour between DW: The God Complex and Closing Time, the writers of the 2005 revival of the series explicitly described the Doctor as around 900 years old, despite contrary statements on TV and expanded media. It is one of the few notable contradictions to established canon in the revival. Although as noted above it's possible to suggest rationalisations (and fans have suggested others ranging from Time War-related trauma to speculation of an unchronicled adventure in which the Doctor was "de-aged" much as he was "aged" in DW: The Sound of Drums and Last of the Time Lords), to date no episode, novel or comic strip has addressed the discrepancy. It's also possible he has been using different 'years' as there are technically billions of definitions of a 'year', all depending on the planet you're situated on.

"He Has No Clue"
A simple and self-evident theory is that the Doctor doesn't know how old he is, constantly travelling backward and forward through time making it difficult to measure one's lifespan at all. This theory has been fanon, and the Doctor occasionally admitted to being unsure of his age in spinoff media. (DWM: The Time Witch, EDA: Vampire Science) This is the theory subscribed to by Doctor Who's current executive producer and head writer Steven Moffat in an interview with SFX: "The thing I keep banging on about is that he doesn't know what age he is. He's lying. How could he know, unless he's marking it on a wall? He could be 8,000 years old, he could be a million. He has no clue. The calendar will give him no clues."

- Steven Moffat, SFX, May 2010 Moffat reaffirmed this on his Twitter blog.

Non-fictional sources
The first non-fictional writing about regeneration, DWAN: "The Phoenix in the TARDIS", stated that the First Doctor was about 900 at the time he was replaced by the Second Doctor.