Roshi is like 350 so Tãopai must have drank it and be like 300+, unless Shen/Tao's parents also had the potion and had their kids hundreds of years apart. Taopai looks a lot younger than Shen and Roshi, so maybe the potion stops you from aging or makes you age really slowly, so he still looks kind of middle-aged.
Did they show Roshi drinking the potion? Was he already an old man?
In the Piccolo Daimao arc, Roshi tells Tenshinhan that he drank the elixir of immortality, but quickly admits that he was lying. There's no reasonable explanation why any of them are that old. Karin is something like 800 years old.