Yes, 20 years is an accomplishment and a clear sign of a good fish keeper. Still, I'm sorry for your loss as even though some would say "they're just fish"...we all know how we feel about our (fur and fins) friends.
I'm sorry for your loss, but that is an excellent pleco lifespan...you did good by your beloved fish! I worry about my 15 year old son's older brother, a 16 year old clown pleco (all of 2.5" inches) going soon, but he seems to be hanging in there. My son will morn, too. Having a fish live that long makes them truly part of the family!