He had to admit, if only to himself—and he would never tell anyone; it was his own private, perfect secret—that he rather enjoyed the appearance of an old man.
Especially around those funny ape-like creatures called humans. They always took things on face value. On his own planet, with his own people, he was considered little more than a young adult, if that. With others, with humans especially, he could get away with acting self-important and dignified. It suited his appearance. To them, he truly was an old man. He really did like calling Ian “young man,” too. Even if the aging of their two species were adjusted, taking into account their idiosyncrasies, the Doctor would come out ten years younger at least. Probably even fifteen. He loved being seen as old and important. Especially when he knew how young he really was.
The juxtaposition was just delicious.