Unless the script has been completely changed it has been stated more then once by Blizz and Metzen that the movie is going to take place in between the events of WC3 and the start of WoW. Which if i recall correctly theres about 5 years in the Warcraft setting there and little to no lore has ever been revealed about what took place then.
I think Chris Hemsworth seems to be typecast as the thor type chracter that seems to suit an Arthas role... but Im sure if done well, anyone could play arthas well... although, I do for some reason see a Billy Connelly roll in there somewhere, maybe even a Gregor Fisher appearance.... obviously neither as Arthas though!!! Mark Stone seems to play the sly villain role well too... perhaps I'm looking too much into UK casting...
Arthas was meant to be 23 when all the events of WC3 went down so most of the suggestions are too old.
The fact he is so young is a big part of the story.
That's a good point, Arthas probably should be played by an unknown younger actor. I would hate it to be like most productions of "Hamlet" where the college-aged title character is played by some white haired old man. Why do they ALWAYS get that wrong?