This is a really productive discussion. Personally, based on zero research but over 3 years of reading here, you have as Sharon says the massive die offs of young fish for bazillion different reasons, then the mortality caused by poor husbandry, followed by the fish owned by people who can actually keep fish alive.
Of the third group, some die anyway despite good care (mystery infections, wen ulcers and sudden death). Tanks are fundamentally different than ponds for many reasons and while pretty much everyone on the forum with tanks has a fish that has lived well past 3, they have had more that died before they had them a full 3 years (although how old the fish actually reached can be difficult to know).
My older fish shows signs of all the things Sharon mentioned - sensitivity to water quality, weight gain and balance issues, and an infection that we had to really work to beat. All of these things hit around the 4 ish year mark. In fact, it is like having a completely different fish now and we are very careful with him and his care.