Does anyone know how long the average tank snail lives for (assuming it dies of old age)?

It depends on the species. Large apple snails can live over 10 years. Small pond, ramshorn, trumpet, etc. snails may live 3-6 years but most die before that when they are eaten or due to poor water quality or something else. Trapdoor snails may live 5-10 years. The average run-of-the-mill small aquarium snail is lucky to live a few years with all the predators and dangers in the average tank. But, really there is no definitive answer to your question. I don't know that anyone's done real studies on this. It's just word of mouth as to how long someone's snail may have lived.