>>> but you pretty much determine the life span of your fish.
At the lower end, that is very true. It is a sad fact that many aquarium fish are thought of as short lived, when in fact, it is remarkable that they managed to stay alive at all in some of the conditions they are kept in.
A fish of good wild stock with adequate genetic diversity, living in it's natural environs and relatively stress free may live for 20 years. The same species, bred on a farm, inbred with all kinds of genetic weaknesses, kept in sub-optimal conditions, stressed with being moved several times and dumped into an amateurs tank with the wrong water and the wrong tanks mates may do well to live 2-3 years, maybe only months.
Small fish live short lives, larger fish longer. That is only a guideline though. Some fish are notoriously long lived. Most great big fish are sold to totally unprepared idiots and last a few months at best. Some Killies are designed to be born, grow, breed and die within a few months, some within a few weeks.
I had a small group of Barbus vittatus many years ago. A now uncommon fish in the trade, but a nice little fish, 50-60mm, never bothers anyone and just goes about it's own business. I have no idea how old they were when I got them, but they were all still there 10 years later.