I thought Ian Holm made a terrific older Bilbo in FotR, but I agree, he wouldn't be able to pull off the younger Bilbo. The flashback in FotR, where he was made up to look younger, was OK for that purpose, but it wouldn't hold up over the course of a whole film.
NOT TRUE! Remember, the ring has greatly slowed Bilbo's aging process. Look at the difference between Bilbo at the beginning of FOTR and at Rivendell. He's 111 years old but looks 50 - the age he would about be in The Hobbit.