McGregor is pretty much in line with everyone else who's played the role, age-wise, and I didn't think he seemed too young at all. In fact, he's older than both Roger Rees and Jeremy Irons were, and the exact age Stephen Dillane was.
I saw it yesterday, as well, and thought him and Gyllenhaal were both excellent. I can see how Gyllenhaal's style might not be to everyone's taste, but Annie seemed a good fit for her. I actually thought Nixon was the most uncomfortable of the three main characters (four if you're counting Max), and didn't connect very much with the character. The first act does have a fair amount of static, and none of the three younger actors do much of anything to distinguish themselves.
As with his production of LOOK BACK IN ANGER, Sam Gold is trying to improve upon something that really doesn't need it with obnoxious stage business.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body