With a lot of the fish I get i'm not 100% sure on their age if and when they die, but if i have a fish its here for the long run, i think i've only ever sold two on (well given), the only way out is via the freezer.
I have kept one HUGE female guppy for about 5 years or something like that and she finally passed. Ummm had a redeye tetra that lived for about 3 years...I couldn't even kill that fish if I had tried.