Marshall's casting for NINE was very hit or miss (Cotillard and Cruz were spot-on, but Daniel Day-Lewis has probably never been so miscast in his life, and why the hell was Judi Dench playing the ), however, I'm pretty sure that Harvey Weinstein had a lot to do with the people who were cast in some of the roles. His casting for CHICAGO was inspired though and he got fantastic, iconic performances from everyone in the cast (Renee Zellweger should have won for this instead of COLD MOUNTAIN). So I really don't think someone like Justin Bieber will be in the cast, but who knows? I think someone like Todd Holland, who played BILLY ELLIOT and will be in the upcoming THE IMPOSSIBLE--which looks great--is more likely to get the role, or even someone like Daniel Radcliffe depending on what age they're looking for.
Not really sure what the big deal is with Streep playing the Witch. The Witch's age is never mentioned but it's pretty clear that she's been around for a long, long time; when she drinks the potion it is told to us that she returns to a stage of youth and beauty but it shouldn't be read so literally, plus the line could simply be cut. This is the fantasy genre so there's a lot of room for change and suspension of disbelief. I will say though that if they're going for an older actress I'd love to see what Glenn Close can do with the role though I know many here hate her singing voice.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"