Doctor Who's Timeless Child retcon rewrote the show's history, but how old is the Doctor now? Showrunner Chris Chibnall promised Doctor Who season 12 would have a lasting effect on the show's lore, and he was right. He revealed the Doctor is actually the Timeless Child, a being discovered by an ancient Gallifreyan explorer named Tecteun, who became the base genetic code for the entire Time Lord race.

This naturally raises a curious question; just how old is the Doctor? There has always been a degree of confusion about the Doctor's age, even before the Timeless Child retcon. In "Doctor Who and the Silurians," the Third Doctor said he had lived for "several thousand years." In "Time and the Rani," the Seventh Doctor claimed to be 953 years old. The 2005 relaunch has generally taken the more conservative approach, with the Twelfth Doctor suggesting he has aged to somewhere around two thousand years old. But how old should the Doctor be considered after the Timeless Child retcon?

Related: Doctor Who Theory: The Doctor Had The Timeless Child's Memories All Along

The Doctor Who season 12 finale, "The Timeless Children," provided one important clue. It revealed the Timeless Child was discovered by Tecteun before the very foundation of Time Lord civilization. That means the Doctor is technically just a little older than the Time Lord race itself. And, fortunately, it is possible to find an approximate age for the Time Lords. As Gallifrey's greatest leader, and a near-immortal at that, Rassilon is probably the most reliable source on this point, and in "The End of Time Part 2," he says, "A billion years of Time Lord history riding on our backs." Thus, as a result of the Timeless Child retcon, canonically the Doctor is a little over a billion years old.

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Of course, the Doctor could actually be far older. Tecteun discovered the Timeless Child on an unknown, distant planet, standing abandoned near a boundary to another unknown dimension or universe. "Tecteun glimpsed the infinite through that gateway," the Master observed, "and beneath the monument she found - a child." There's no way to know just how old the Timeless Child really was at this point, nor even if there had been prior regenerations.  The Time Lords are known to be able to regenerate someone to childhood, so this could have happened before.

Still, this indicates the scale of the Timeless Child retcon. Before Doctor Who season 12, estimates of the Doctor's age varied wildly, but there had never been any indication she could be a billion years old or more. Viewers have only seen a fraction of the Doctor's lives, meaning great chunks of the past have yet to be revealed.

More: Classic Doctor Who Set Up The Master's Timeless Child Discovery