While the Doctors are obviously going to change from time to time, all the Doctors on the reboot tend to have be showing a pattern when it comes their doctor selection. The 9th doctor was a little past middle age and the third oldest of the four new doctors. While he was old for the new doctors, he is probably considered young for the doctors in the old Who.
I believe the producers saw the response to the younger 9th doctor and decided to continue with the trend of an even younger doctor by choosing David Tennent to be the 10th doctor. While I my knowledge of reactions to doctors extends to that of our class, it would appear that many peoples favorite or top doctors is Tennent. Once again I believe that the producers saw this increased positive reaction to a younger doctor and took it one step further. The brought on the 11th doctor who is Matt Smith who is the youngest doctor to date and while I have heard people express that they do like Matt Smith, I have heard people make jokes about how young he is and that he is a “child”. A lot of this push back on the youth fullness of the doctor I believe comes from the “Old Who” fans who believe the doctor should be older. After some pushback however, I believe that the producers went ahead corrected heavily be choosing Peter Capaldi who is much older than any of the last couple of doctors.
While I haven’t seen any of the episodes with Peter Capaldi, I believe there are no relations with the doctor and anyone, unlike any of the other especially like Smith and Tennent. That is why I believe the that with the new doctor being selected they will go back to younger doctor so that they can incorporate that component back into the show.