How old is Bilbo in the Lord of the Rings?

Answer 1

The Maia itself, known to us as Gandalf, is older than the world. In the shape of Gandalf he arrived in the Grey Havens in Middle-earth about the year 1000 of the Third Age. He is killed in that form on 25th January 3019 T.A., and returns to it on 14th February. Gandalf leaves Middle-earth in autumn 3021 T.A., and from then on nothing more is known about him. So we do not reliably know how long he keeps being bound to his "Gandalf" form, or which form he chooses to generally use.
As you can see, we cannot say how long "Gandalf the White" lasted beyond the 2 1/2 years before he left. Unless you mean the "Gandalf" form overall, which would then be at least a thousand years, or the actual spirit/being regardless of form, which would be older then the created world/universe.

Answer 2

In chapter 1 of Book 1 of the Fellowship of the Ring, Bilbo is 111 years old. By the end of the Return of the King, when he sails to the West, Bilbo is 131 years old.