900 was a number.
Late in the life of the Fifth Doctor, he stated that he was "pushing 900." (AUDIO: Omega) The Sixth Doctor said he was 900 years old while travelling with Peri, (TV: Revelation of the Daleks, The Mysterious Planet) "not a day over nine hundred years old" when threatened with death by stoning. (PROSE: The Mysterious Planet) He told Constance Clarke that he was over 900 years old. (AUDIO: Absolute Power) Flip Jackson later called him around 900, but she was not sure on the exact age. (AUDIO: The Middle) By the time he was travelling with Melanie Bush, he had blown out the candles on his 900th birthday cake. (AUDIO: The One Doctor) By the end of his seventh incarnation, the Doctor kept a Nine Hundred Year Diary. (TV: Doctor Who)
After being slapped by Jackie Tyler, the Ninth Doctor noted that in "900 years of time and space" he had never been slapped by someone's mother. Immediately after he made this comment, he told Rose Tyler it was "[his] age." (TV: Aliens of London) He later considered himself to have had "900 years of phone box travel". (TV: The Empty Child)
Having said he was 903 years old earlier, (TV: Voyage of the Damned) the Tenth Doctor claimed to be 900 in Nevada. (TV: Dreamland) While confronting a rabbit in Elizabethan England he briefly believed to be a Zygon in disguise, he said that he was 900 years old. (PROSE: The Day of the Doctor) Another account claimed he gave his age as 904. (TV: The Day of the Doctor)
The Eleventh Doctor spent a total of 900 years on Trenzalore. (PROSE: Tales of Trenzalore: The Eleventh Doctor's Last Stand)