Episode 63 of "Futurama"
s05e07 Teenage Mutant Leela's Hurdles

When Fry and the Planet Express gang realize that at 161 years old, the Professor has become too crotchety to handle, they decide he needs to be "youth-a-sized". Traveling to an asteroid, they register the Professor at a posh resort where their full-body bath in searing hot tar is guaranteed to turn back the clock. Through a series of unfortunate accidents, the bath covers not only the Professor but also everyone else and all are affected by the treatment. Back at Planet Express, a now 53-year-old Professor tries to keep a now 13-year-old Fry and other kids under control while looking for a way to reverse the effects of the tar treatment. Meanwhile, "the kids" all react to their newfound youth, including Leela, who decides she could have a taste of the childhood she never had as an orphan.