***At 35, Jude Law would be rather too young to play Ozymandias.***
Well I reckon the angle they're going for with the young casting is for the flashbacks/early years etc., we will need to see the heroes when they are young so they need a youngish cast. It's easier to make young actors look old with make-up/fx for when they are playing the older versions of their character than to try and make old actors look young for earlier parts of the timeline.
I'd rather Jude Law didn't get the part though as I think Christian Bale would be a better choice, he's an actor who can portray both intelligence and physical strength particularly well whereas Jude is a bit of a pansy.