Bilbo was 50 in the Hobbit.
There ya go! In the prologue to Fellowship, they showed a bit of Bilbo finding the Ring in Gollum's cave. Ian Holm looked fine there, so he could definitely do it!! It would be a fitting close to the series to have him play that part. I don't know if it would be good for any of the others, except for maybe Ian McKellan and the Elves, since they're immortal, to be in it as well. Might be disconcerting to see the folks we know so well in their LOTR parts to be playing in "The Hobbit" unless they're made up to be older people and relatives of their LOTR characters.