I have no problem re-imagining the age of the Gods, truth told. The primary reason we imagine Gods like Zeus to be old is because, well, he's an old God and he has fathered other Gods and Goddesses. Therefore, he must naturally be old, right?
But why would an immortal being become old? In addition, a lot of Greek art used actual rules and calculations to depict the human form, creating a "perfect body". Why wouldn't the Gods, in contrast, always have the perfect sort of form? So casting young actors in the role of the different Gods didn't quite bother me.
Plus, I'm pretty much a fan of the guy who played Zeus. He keeps getting roles that appeal to my geeky heart and he executes them well. Leto Atreides in Children of Dune, Mr. Tumnus in Narnia, Professor Xavier in X-Men: First Class...yeah, he's good in my book.