He had grey in his hair. Grey! After getting increasingly younger bodies for the past several regenerations, he was suddenly beginning to look old!
Alright, so before this regeneration he already looked old, but that was because his body WAS old. He’d had that body longer than any of his previous ones, and it had aged accordingly with the passing years. This one, however, was new; it should look young! He should feel rejuvenated, not like a crotchety old man.
Clara said the salt-and-pepper look was distinguished, that it suited him, but what did she know? She wasn’t going grey!