450 was a number.
At the time of his regeneration, the First Doctor was 450 years old. (PROSE: Iceberg) The Second Doctor later told Victoria Waterfield that he was "about" 450 in "Earth terms". (TV: The Tomb of the Cybermen) Upon reuniting with the Doctor, then in his fourth incarnation, Drax recalled that he was on the tech course with the young Doctor together 450 years prior. (TV: The Armageddon Factor)