I really liked the actor as Stannis.
At first I was a little put off. I kept thinking that he was NOT Stannis, but his performance won me over.
I think he captured Stannis' humorless disposition while bringing the unintentionally hilarious Stannis to life. Some of the best guffaws I got from the books came from Stannis.
Even though the actors playing Jorah and Stannis don't match their characters descriptions, I see them now when I read the books.
For Jon, however, he is still a mental composition. The actor has yet to "stamp" the character for me.
I totally hope that Stannis does look older. In the books, I got the real sense that Renly's death combined with whatever Mel was doing was sucking the life out of Stannis. He became more skeletal to me as the books progressed.