This is an old post, but just to reiterate, 3 of my 4 silver dollars are at least 13 years old. I got them from a pet store as young adults 12 years ago. The previous owner had "surrendered" them and I got them for free. I figure they must be at least 13. The fourth is the offspring of a mating pair I had. (The mom died of a tumor at the ripe old age of 10, I still have the dad). They had about 14 surviving little ones that I gave away and just kept one of them. I started out with 6 SDs, but two of them died prematurely, one of a tumor and the other one (I assume) of a broken spine, when it jumped into the top glass in the middle of the night and died instantly.
This is an old post, but I am a 'silver dollar freak', so for what it is worth...
Silver dollars live long lives, several decades.
However, I am sure there are important differences among the various general and species. In addition, longevity in captivity (if well kept) is almost certainly longer than in the wild, due to the absence of predators, protection from external and internal parasites, etc.
I can only speak of direct experience with Metynnis dollars. However, the longest life I heard of was about a specimen of Mylossoma duriventre, reportedly living to 40 years in captivity.
I have 3 specimens of Metynnis maculatus who are 26 years old.