Matt Smith is a 26 year old actor who has performed with former Doctor Who companion, Billie Piper, in a few of Philip Pullman's Sally Lockhart mysteries adapted for the BBC and also shown on PBS's Masterpiece Mystery.
I've seen the first installment, The Ruby in the Smoke. Smith plays a scruffy thirteen year old who is befriended by Sally in her quest to find out the truth behind her father's identity and a mysterious ruby. He appeared also in The Shadow in the North and only briefly in The Tiger in the Well.
The last three doctors seem to be getting younger and younger, which makes me wonder if the the 12th and final regeneration will leave the doctor as an infant - not unlike Benjamin Button? But I hope that wouldn't be the end of Doctor Who. Perhaps he would start all over again, this time growing older with each regeneration - who knows?
