James McAvoy as Bilbo Baggins?

[Lalaith]

Hobbits don't mature any more slowly than we do

Yes they do. They officially come of age at thirty-three, rather than 18 or twenty-one as we do. They live for around 120 years rather than four-score and ten.
So Bilbo is around 17 hobbit-years past coming of age. McEvoy is thirty. So I think that's not too bad at all, nowhere near as inappropriate as Eliljah Wood and co.