All I can think is how Vastra believes some subconscious part of The Doctor wears a young face to fit in and yet instead of getting another young face, he gets an old one now. So if Vastra’s theory is correct, why? Eleven didn’t want to be old and gray. But…he’s still got River’s words in his head and in his hearts: “When one’s in love with an ageless god who insists on the face of a twelve-year-old one does one’s best to hide the damage.” He stopped trying to fit in and instead tried to become the face he thought his wife would want.