Marty makes a couple of good points.
First, optimum fish ages depend greatly upon optimum conditions. That's just tough for most of us to accomplish in a tank. But, we try. (And have fun doing it.)
Second, I was thinking the same thing: those fish were probably stressed.
I've read about "old tank syndrome", in which the tank gets neglected, but because of pH the NO3, NO2, and NH3 don't affect them quite the same way as they would in a normal pH setting. Then, suddenly better care gets taken of the tank and the fish begin to die. Why? Well, the pH gets straightened out and the "bad stuff" in the tank starts to affect the fish, or at least stress the fish. (Simplified, but to the point.)
Is that what happened here? Could be. I'm not saying you purposefully neglected the fish, or that missing your monthly cleaning was neglectful. But, it might have been just enough to set off this chain of events.