Here's a crazy theory I concocted a few years back: the mud, doesn't deage you a set number of years, but a percentage.
Fry start out as 27-28 years, and right after he comes up, his age is 14. Leela looks 14-15 too. Amy looks more like around 11, or something. All indicate a 50% loss of age.
Hermes have a son, Dwight, who looks 12-13, making Hermes in his mid-thirties. Half that, and he'll be around 17. Maybe a bit on the high side, but Hermes does look slightly older than Fry and Leela in this shot:
Farnsworth went from 160 to 53, a loss of 66%. Could correspond nicely with him sitting in less-concentrated mud for a few minutes prior to the cave-in.