The chances that someone will get a fish that is close to it's natural end of life is rare...very rare. Mostly, because if someone has had this fish long enough to get it close to that point, they are very unlikely to get rid of it........especially if they have gone the route to take such good care of it to get it to old age in the first place. Unfortunately, most fish in the hobby world, die from issues we create. Most probably even die within the first few months of captivity from diseases and parasites, because too many people strut their way into the hobby without a clue, fail to quarantine properly and think crypt or other maladies will not happen to them. The stats are this 40-50% of the fish collected for the marine ornamental hobby, are presumed to die before they get into the tanks of the LFS. So, collection, transport, and wholesalers see most of the losses. Also, only about 10-15% of all marine hobbyists stay in the hobby for more than 2 years.......that is how great the turnover is. Many reasons go into this.....frustration with how hard it can be to be successful, depression from not quarantining, adding that last fish and killing off a whole tank that was doing awesome, and expense.....especially in these days and times. So, basically, you pose a good question, but the true answer should lead you to know that getting a fish suffering from senescence in the hobby has such a small chance of occurring, that you need not worry.
If you knew how many fish are collected from the wild and shipped to the US and Europe, you would realize that the death toll is huge..........or every household would have to have a tank to house all the fish that have been caught and shipped. As of 2001, the US accounted for about 80% of all the marine ornamental hobby fish. Europe has started to gain on us though and I don't know what the numbers are for 2010.....and I must admit, the europeans tend to be way better aquarists than we are........they read more about it before they get into it because it costs almost double there what is does here.
Hope that helps.
