Disclaimer: Don't own Doctor Who
The Doctor told people that he was 1200 years old. To be fair, he usually also explained that he was probably lying, or that he had made a miscalculation somewhere along the long and winding road that had been his life journeying through time and space.
The truth was, he was lying – he just didn't know it.
When he had stolen her, he had been 200. They had both known that – for, at that point in his life, the Doctor had not actually travelled in time before. He had crosses the cosmos, indeed, making it to the Medusa Cascade when he was a mere 90, yet he had never once entered the Time Vortex. His life up to that point had been linear; he often described it as boring.
So, when their journey together began, they were both aware of his relatively young age; maybe not young compared to the many strays he had picked up over the years, but certainly young in the life-cycle of a Time Lord, and compared to the age of the universe, he was still almost an embryo.
Yet when he had first skipped through time – going forwards first, as everyone did – he had known from that moment on that there was no point in trying to keep track of his age. He had told Susan the same thing, looking down at her with those grandfatherly eyes and telling her that she would never again be able to say with exact certainty how old she was. Neither of them had minded, and neither of them had been aware that the Doctor had been lying.
In truth, the Doctor did not know how old he was. There was only one person in the universe who did, and she made sure to keep it a secret.
Despite being manufactured for a life stuck in a museum – never seeing anything other than the same four walls that surrounded her and the faces of the other Gallifreyans as they wandered through the exhibit hall, the children gazing up at her with wide-eyed wonder, delight and ambition, turning to their parents and telling them that, one day, they were going to get their hands on one of those and go off and see the universe – the TARDIS had been made in perfect working order. She had the ability to transverse across parallel universes, to jump through the Time Vortex and take her passengers anywhere and anywhen they wished to go.
She also had a calculator. It was unlike any other calculator that could be found anywhere in the universe (other than in other TARDISes), for it was one that kept track of the ages of her passengers. She knew exactly how old each one of the strays had been when they had walked through her doors into their normal dimension for the last time, leaving her thief behind just that little bit more lonely and heart-broken – not that he would admit that, not even to himself. No matter how long they travelled with her, she would calculate their age each and every time they stepped through her doors, and that included the one person who had never left: the one constant in all of time and space.
It had been a long, long time since she had calculated the Doctor's age in any term that would have been meaningful or understandable to anyone who had been involved in her making. When she had rolled off the assembly line, the calculator had only understood Gallifreyan years; when the Doctor had stolen her, she had initially known him as 30, for Gallifreyan years were much longer than those of his beloved Earth's. During his exile in his third incarnation and his continued efforts to fix the Dematerialisation Circuit, he had tinkered with many aspects of the TARDIS' controls, including recalibrating her to think in terms of Earth years, rather than Gallifreyan years. She went back across her entire records of the Doctor's age one he had done this, correcting and changing them to fit with her new understanding and his apparent preference. He usually gave his age in terms of Earth years anyway, even to those who had not originated from that blue and green planet.
So, from that point on, she had waited for him to return from every adventure and made a note of how much Earth time had passed, to add on to the previous number stored within her Data Core.
She had often considered telling him that she knew how old he was; he didn't seem to be aware of the fact that her calculator existed – he certainly hadn't recalibrated her with the intention of finding out how many times the Earth had circled its star since he had been born. She almost had on a few occasions, back before the Time War – back when he had been relatively happy.
Yet the Doctor's life had been a hard one; he had suffered losses and heartbreak more than anyone deserved to, and it never seemed to be the right time to reveal that information to him. Then the Time War happened and he was never the same after that, always sad in one way or another despite the smiles that he hid behind.
It was after the War, especially in his tenth incarnation, that he had seemed to begin to realise just how old he was. She almost thought that he was ready to find out, but then he still told people that he was merely 900-and-something; he was still lying, both to himself and to others. And he still had no idea.
The years went on and the Doctor got older, and he felt older too. For all the childish glee and energy he had gained with this new, more youthful incantation, he was still an old soul, floating around the universe with no one to match his age. Even so, he still considered the Roman to have lived longer than him: that 2000 Earth years surpassed his 1000, even his 1200. He had no clue how big that lie was.
Only one person in the universe knew exactly how old the Doctor was, and she had known for a while now that she would never tell him. He underestimated his time in this universe, but the number he gave to people was comforting; he wasn't sure that it was accurate, and she knew for a fact that it wasn't, but she would never tell her broken Doctor that. For she feared that, if she did, then he would break beyond repair.
