I just completed watching season 1 and season 2. I really liked the casts specially Claire Foy, Matt Smith.... They were really getting deep in to their characters.
But from Season 3 these actors are changed which is not at all expected.
I think that a person's face resembles to their young aged faces no matter how old they become.
So Claire Foy and Matt Smith should have been portrayed in all other seasons by proper make ups so that they could have looked old.