The chart above shows the age of Thorin and Company, including Bilbo and Gandalf, at the time the quest to reclaim Erebor takes place. In Peter Jackson’s adaption of the Hobbit they have changed the age of the characters quite a bit, most notably by having Thorin appear to be younger than Balin and Ori being the youngest of the Dwarves.
Thorin, born in T.A. 2746, was 195 years old at the time and the oldest of the Dwarves in the Company. In the movies his age has been changed and he is portrayed as a fairly young looking Dwarf. The fact that he is present at the Sacking of Erebor means he must be at least 171 years old (+ his age at the time of the sacking) in the movies.
Balin, born in T.A. 2763, was 178 years old and the eldest of the Dwarves after Thorin. In the movies he is portrayed as an old Dwarf, very likely older than Thorin. He must also be slightly older than in the book where he is 7 years old at the time of the Sacking of Erebor.
Dwalin, born in T.A. 2772, was 169 years old at the time.
Óin, born in T.A. 2774, was 167 years old at the time.
Glóin, born in T.A. 2783, was 158 years old at the time.
Fíli, born in T.A. 2859, was 82 years old at the time.
Kíli, born in T.A. 2864, was 77 years old at the time.
The age of Bifur, Bofur, Bombur, Dori, Nori and Ori has been left unknown. However, we know that Balin was the eldest of the Dwarves after Thorin and that Fíli and Kíli were the youngest “by some fifty years”. This would put their age between 132 and 178 years at the time of the quest.
Bilbo Baggins, born on 22 September T.A. 2890, was 50 years when they set out on the quest but turned 51 before they reached the Lonely Mountain.
Gandalf is just really, really old and has been around since the beginning of the world.