this is what i found on the subject
corals are colonial, cloning species with very slow growth; generation times may last decades or centuries. Individual polyps may die of old age, and there are certainly less hardy species around, but it is conceivable that some colonies will succumb only to disease, chemical, or physical destruction. Some coral species may last for 20-40 million years; some single generations seem to persist indefinitely. However, maximum genotypic longevity is probably limited by the establishment of present sea level about 8,000 years ago. The oldest reef found to date is 8600 calendar years old.
"most corals do not seem to experience physiological senescence and individual colonies have the potential for indeterminate clonal growth." in some cases, modern colonies are still composed of the original post-pleistocne colonists or only a few generations removed. "...once certain clones become dominant, their generation will tend to be prolonged indefinitely and their genes will persist in the population until the ancient genotypes die, something that may require total destruction of the habitat."
generation time and the quaternary evolution of reef-building corals
http://www.jstor.org/stable/2400500?seq=...
http://www.statoilshop.no/statoilcom/svg...
a study of deep-sea corals (which make up two-thirds of all species) identified living gerardia sp. And leiopathes sp. Specimens that were 2,742 years and 4,265 years, respectively.
Extreme longevity in proteinaceous deep-sea corals
http://www.pnas.org/content/106/13/5204