That page is hardly authoritative; to the point of being junk. Having said that i doubt there is much reliable data available. Most people are poor fish keepers or have unsuitable conditions for optimal (or even good) longevity.Never kept a pleco before but according to this page, they say 10 years. So your pleco must have some amazing workout or health care routines!
However if you really want a single fish for the rest of your life I'd go with a group of clown loaches in a 200 - of course if you have a 2000 gallon aquarium you could get away with a single pacu.Yes, bigger fish like that are likely to live to at least 20 if cared for well. Even small fish can randomly live to extreme ages occasionally. My record used to be a solitary silver hatchet which I had for 17 years, but I think that's been beaten recently. Last month my last remaining red scissortail died. Unfortunately I don't have a note of exactly when I got those fish but I know I had them in a tank that I replaced in 2007, so that was over 17 years ago. I nearly always seem to end up with one fish from a group that far outlives the rest.