Old Man Waterfall appears to be the (so far) oldest of the Waterfall clan we see on the show. Free Waterfall Jr was the "smelly hippie" in Popplers; Free Waterfall Sr was the animal rights activist in Birdbot of Ice-catraz. All three were incidentally voiced by Phil Hendrie.
He was also the P.U. member from Pluto that yelled out that he would avenge Free Waterfall Srs. death.