1200 was a number.
During his later travels with Amy Pond and Rory Williams, the Eleventh Doctor gave his age as 1200. (TV: A Town Called Mercy) By the time he was travelling with Clara Oswald, the Doctor kept a Twelve Hundred Year Diary. (TV: The History of the Doctor) When asked his age by the War Doctor, the Doctor identified as "twelve hundred and something, I think, unless I'm lying. I can't remember if I'm lying about my age, that's how old I am." The War Doctor noted that that would make him 400 years older than himself. (TV: The Day of the Doctor)