: : : In The Hobbit, it also says that dragons live practically forever unless they are killed. So perhaps the theory that dragons are inhabited by spirits is correct.

: : : -Valgus

: : From page 213 in the Silmarillion:

: : "And now he stood alone, for the few that followed him had fled. But in that moment Glaurung issued from the gaping doors, and lay behind, between Turin and the bridge. Then suddenly he spoke, by the evil spirit that was in him, saying: 'Hail, son of Hurin. Well met!'"

: : We see here that dragons (assuming that all were like Glaurung in this matter) do indeed have spirits...

: : From page 116 in the Silmarillion:

: : "Again after a hundred years Glaurung, the first of the Uruloki, the fire-drakes of the North, issued from Angband's gates by night. He was yet young and scarce half-grown, for long and slow is the life of the dragons, but the Elves fled before him to Ered Wethrin and Dorthonion in dismay; and he defiled the fields of Ard-galen."

: : Here we see that the dragons did age and it is likely that they died of old age. Of course, this (dragons died of old age) is only a speculation and my opinnion.

: : Maedhros

: But if they did die of age, it would IMO take several thousands of years.
: -Foradan

Of course, Glaurung lasted from the flight of the Noldor till the death of Turin. I'm not exactly sure of how long that is, but Glaurung wasn't anywhere near death of old age.

Maedhros