Speaking as a public school victim, I disagree with you about Tennant (let alone that his name is that of a great landed house). And David Eccleston lasted how long?

Holmes and Watson in A Study in Scarlet were being introduced as being at the beginning of their respective careers. None of the later stories, except for Gloria Scott and The Musgrave ritual, shows either of them as very youthful. And Sidney Paget gave Holmes a visibly receding hairline.