So I'd assume that dragons live for a long time — at the very least, around 500 years or so, and that's just me guessing off the top of my head. There is a very clear distinction in terms of attacks and stats, however, between "adult" dragons and "ancient" dragons. When in its lifespan exactly does an adult dragon become classified as ancient? Is it when it's on the brink of death? Is it after a certain number of years? Is it once the dragon gains a certain amount of infamy, due to its terrible deeds or glorious renown? And does a dragon ever get so old that it's beyond ancient — as in, teeth falling out, dentures, bifocals, etc.?
This is answered on page 86 of the Monster Manual. A dragon is classified as ancient when it reaches the age of 801 years or more.
As for anything beyond ancient, well, there are no current stats for a dragon beyond the status of ancient (that I'm aware of) so that would be the purview of the DM.