Middy arrived on Hydaelyn pretty near "the beginning of time" with the eggs of the first brood.
This was sometime after the sundering, which was about 12,000 years ago.
Middy was already old enough to be a great dragon and travel across space, so it's safe to say that he's way older than 12,000. By several thousand years at least.

The First brood hatched shortly thereafter and are likely between 10,000 and 12,000 years old. Probably leaning towards 12,000.
All other dragons are younger.