Find me someone who really has a 15 year old discus not something like I heard this person or that person. Over at simplydiscus most people keep their discus 2-3 years. The average people who buys discus keep it under year. It is not a hard fish to keep but it is a fish that demands your time. If you are able keep discus alive for more than 3 years I would say you are good fish keeper.
I really don't know how long the average discus can live and I haven't seem any post stating facts to prove they have kept 10 years or older. The average time to grow a discus to 6.5" takes about 1 year. When my fish died I think they were around 6 years old.
If you are thinking of buying small discus and growing them big. I wouldn't recommended it. I tried 3X 50% water change weekly on 4" fish and they only grew to 5.5" with 84F temp. It is hard but I am not saying it is not impossible. I was told many times discus farms does daily 90% water change and power feeding. It is cheaper to buy adults.
This is a link to one of my discus tanks before they went belly up. Very pretty fish.