I would say size really, if you research the fish and know its growth rate you should be able to get it within a good frame. 2-4 years isn't correct from what ive witnessed on others.
My dad has had a Tang for 6 and has had at least 3 others that are 5. He told me when I started mine, good water flow to keep them strong, meaty supplements to keep them strong, and they will last if the water stays good. Hes been in the game for at least 8 years and his tank is amazing. My true idol. His dad was an ol schooler though. No skimmers and no filters. WC and lots of them and LR. I remember him saying. "Why do you need a machine, the ocean dosen't need a machine!" he had great reef tanks also. Ill see if I can pull up some of his old polaroids and scan them.
Anyways im rambling.