Question: So far as I can tell, there is progression of time in the series and there being a continuity and history, now stretching 23 years. So why do none of the characters ever age? Maggie has been a baby for 23 years now.
Question: Are the characters aware they don't age?
Chosen answer: In general, no. However, in the episode "Behind the Laughter," Lisa comments about how Homer fed her anti-growth hormones to prolong the run of the show. Chronological events do occur, such as birthdays (which all of the characters have had in various episodes), and several episodes include flashbacks and peeks into the future. However, for the most part, the characters remain in a chronological stasis.
Question: How did Herman lose his arm?
Answer: He stuck it out of a school bus window, and an oncoming truck subsequently ripped it off.
I've heard about that but in one episode there was a flashback and it got knocked off by a car so which is correct?
Answer: In the episode To Cur with Love it was revealed that he lost his arm while trying to hail down a car. His arm was ripped off by a dog catcher van.
Chosen answer: No character has really aged. The show is on a sliding time line. Bart will forever be 10, and Lisa 8 for as long as the show is on the air.
MasterOfAll
Ling Bouvier is one that has aged.
Dan23