the problem is that they have to start out with someone who can pass for 28...
Personally, I don't see Marsters as Harry. I've never listened to the audiobooks, but even if he does have the right voice, I don't see him looking like Harry.
That said, they can do a lot with makeup...
How about Donald Sutherland as Arthur Langtree?
I started with the audio books, He does an excellent job with all the characters. I travel alot, and there is something satisfying about being able to listen to your book while on the road, especially when you have precious little time to read. I have listened to a bunch of books, and he is by far in the top five of people who read these stories. I recently listened to ghost story, It was narrated by someone else ( I forget his name ) But though he did a credible job... It just wasn't right. The whole presentation was... just one off of what it was supposed to be. Like that phone call from someone you know well, and for some reason you just can't recognize the voice, like it is not them.
I have no Idea what he looks like, and I am not giving him a blessing to play Harry on TV, but what ever they are paying him to play him on radio... it is well worth it.
As for TV/Movie Casting... I think that Liam Neason can play any number of characters in Dresden Universe and get away with it. Top Three I would choose for him are.... Micheal, Morgan, and The Merlin... in that order