I like Daniel Radcliffe but he's Harry Potter. It's going to take a few years and some radically different films out of him before I'll see him as any other character. And, yes, WAY too young.
Besides, he's going to be tied up with Deathly Hallows till next year some time so depending on when Hobbit started filming, he might be available.
Tennant would be good! I like him, thought he made a great Doctor. McAvoy is a good actor also and I could see him in the role, I think.