904 was a number.
While travelling alone, the Tenth Doctor claimed to be 904 years old. (PROSE: Number 1, Gallows Gate Road) While confronting a rabbit in Elizabethan England he briefly believed to be a Zygon in disguise, he said that he was 904 years old. (TV: The Day of the Doctor) Another account claimed he gave his age as 900. (PROSE: The Day of the Doctor