Most corals need a hard surface, like rocks, in order to attach and grow. Some can live in soft sediment. Some shallow water corals can grow 10-20 centimeters per year. Most established shallow water coral reefs are 5,000 – 10,000 years old.
Deep-sea corals grow very slowly. Some species only grow about 10 millimeters per year. A colony of black coral was determined to be 4,265 years old and are the oldest known marine organisms. Since corals can also reproduce asexually and continuously build colonies over time, some deep-sea coral reefs have been actively growing for 40,000 years!