The next actor to play the lead in Doctor Who was in fact the youngest yet to be cast in the role. Matt Smith was aged only 26 at the time.
The Eleventh Doctor is particularly childlike, allowing him to connect very well with children (the first person he meets after his regeneration is a seven-year-old Amy Pond; the adult version would become his first companion). In fact, this persona is quite ignorant of correct behaviour, awkward in social situations, and seems incredibly slow in realising things (the audience are often a step or two ahead of him)! It's said to due to his advanced age (mentioned in one episode) or being too stupid to remember details (as mentioned in another). He is secretive, keeps vital information from his companions, and then blames himself for ruining many so many other lives.
And yet... this Doctor can get excited over anything different and yet interesting. He has a powerful sense of right and wrong and a determination to do what's right. And he knows that bowties are cool.
Matt Smith has appeared in 37 stories (42 episodes) and will hand over the TARDIS keys to Peter Capaldi after this year's Christmas special...