Corals don't die of old age, depending if you mean each individual polyp, or the entire colony. A basic lesson of how a reef is formed in nature: All the SPS corals spawn at the same time, releasing 1,000's of eggs and sperm cells into the water. If the corals are lucky, a sperm will hit an egg, and fertilize it. Then, the gamete (fertilized egg) will eventually land on something hopefully hospitable as far as the coral's needs. As that coral grows into a big colony, the lower sections that the coral grew from will be shaded out, and thus will die. This will continue, and the dead sections of coral will become rock. So, while the coral as a whole colony of polyps will continue to live on, each individual polyp will die eventually.