User blog comment:27finnsprincess/SusanStrongXFinn/@comment-5094574-20120607200650

Here is something for you to think about:

Due to The Mushroom War, normal humans were turned into Humanoids, however, in the episode "Beautopia" the city resembles a war bunker. The "fish people" might have been normal humans at one point, but turned due to radiation. Susan might have had exposer to it but didn't get the full effect and her age remained isolated. Besides according to evidence the war probably took place over 900 years ago but radiation kept everyone in isolation so they couldn't age.

Theory^