There's an episode where they pretend the Simpsons are a real family and do an "interview" of them since they gained fame. In that episode, Homer says something like "people think that I use growth hormones to keep the kids young, but how am I supposed to fit all three pills in their coffee?" At the time, I believe this was supposed to (jokingly) explain their everlasting youth.

(I tried to find this clip on YouTube but can't remember the details well enough to find it)