With the Twelfth Doctor now midway through his inaugural season a common complaint about Peter Capaldi’s version is that he seems to have forgotten much about humans and life on earth. Whereas the Ninth, Tenth and Eleventh Doctors were all experts about the human experience, the Twelfth seems confused by even the most run of the mill detail such as aging and body type.
A big reason for this departure may be the fact that prior to regenerating into the Twelfth Doctor, the Eleventh Doctor spent the last 900 years of his life away from earth. He was in the middle of siege against the combined forces of all of the Doctor’s enemies including Daleks, Cybermen, Sontarans, and Slitheen, at Trenzalore. He did not get to travel in the TARDIS and other than Handles, did not have the company of a companion.
This experience likely had a large effect on the new Twelfth Doctor as he is now nearly twice as old as he was previously. The forced solitude while under attack could have also given the last Time Lord a kind of PTSD as well. The Doctor could have been shaken by the memory of so many centuries of constant war and fighting. This could have resulting in his apparent confusion with many pieces of human history.
Additionally, one has to wonder if the regeneration itself might have had complications. No Time Lord had ever been given a new set of regenerations before. It is unknown whether there are diminishing returns on the quality of the regeneration or if there are other side effects of regenerations for a 13th time. After all when the regeneration began, the Eleventh Doctor shouted, “We’re breaking some serious science here, boys!” This implies that he was going into unknown territory.
Perhaps that is why this Doctor is older, a combination of the Doctor natural aging while at the Siege of Trenzalore and possible side effects of a second regeneration cycle. Older people often forget things more frequently than younger ones, so that may be why the Doctor no longer seems like the expert on human history he once was or has trouble with human aging or telling people apart.