And so Tennant’s hanging up his TARDIS key. He will be an ex-Timelord. Apart from the 2013 and 2023 reunions with Messrs Baker and McCoy wheeled out in their bath-chairs of course.

It’s most likely that he’ll act his last on either Christmas Day or New Year’s Eve 2009. Of course, for those followers of the ‘Laverne As Doctor’ school of (mathematical) thought, they know that he’ll last until 17th April 2010. A ‘last gasp’ five minutes reprise at the start of an episode, or a full blown ‘transition’ episode? It remains to be seen. But right now people seem more interested in just who will be taking over, rather than when.

First, a bit of maths. Because you knew this bit was coming.

The eldest actor to step through the doors of the TARDIS for the first time was William Hartnell back in November ’63. He was 55 years and 10 months old. Second eldest was the 50 years and 6 month old Jon Pertwee. The youngest have been Peter Davison back when he was just 30 years 9 months and, of course, David Tennant at 34 years and 7 months. The average age of a Timelord on his (her?) first voyage has been 42 years and 2 months. This is something that, I think, we should take into account when choosing the next regeneration, especially given Steven Moffat’s (admittedly ten year old!) comments on the subject: “I don’t think young, dashing Doctors are right at all. He should be 40-plus and weird-looking — the kind of wacky grandfather kids know on sight to be secretly one of them.”