The Simpsons has been running for a long time, how does the series handle the character's aging? There's an annual Halloween episode, so we know the years pass in Springfield, but do the characters age at all?
The Simpsons utilizes a floating timeline to explain how the world of the show advances in time while the characters remain the same age.
A floating timeline (also known as a sliding timescale) is a device used in fiction, particularly in comics and animation, to explain why characters age little or not at all over a period of time - despite real-world markers like notable events, people and technology appearing in the works and correlating with the real world.
The show always takes place in the current year and the characters will always remain the same (general) ages except in certain episodes that show flashbacks or someone's future or where the plot simply requires a birthday, etc.
As a result, there's actually very little continuity between the episodes themselves and plenty of things that just don't make sense if you look at the series as a whole. Basically, the writers do what they want, and if you ever notice a problem you can just assume a wizard did it.
The following Wikia link says -
The aging of characters in The Simpsons has been a subject of common fan debate. The passage of time is clear, but characters only show minor, if any, signs of aging, despite openly saying that years have passed. The children also remain in the same grades at school. The writers and character designs seem to ignore the issue of aging.
It's not the only cartoon where age doesn't change with time. There are so many others too that don't need to be mentioned. The given wikia link also tells you the
Common Theories used for the non-aging issue of Simpsons .
Common Theories Used to explain this are -
One common theory is that, like in many cartoons, the characters never age and are trapped at the age of their first appearance. However, Homer, Bart, Lisa and Maggie have all had episodes involving their birthdays.
Another common theory is that, because only one half-hour episode airs approximately each week, the flow of time in The Simpsons is considerably slower than in real life. This theory is supported by the apparent, though minimal, growth of the characters, as well as the noted marks of time. However, this causes a problem with episodes such as All's Fair in Oven War, when the kitchen takes 2 years (in the Simpsons-verse) to remodel, but no-one has aged.
Though the above theories are accepted, they fail to address the fact that as the years go by, it becomes apparent that the characters evolve through time. Homer and Marge fell in love in the 1970s in early seasons, and now it is said to have occurred in the 1990s. It may be that as time progresses, the characters present, past and occasionally future selves evolve into consecutive timezones.
An example is if the viewer is told in an episode that aired in 1990 that Homer was born in 1963, then in a 1991 episode he will have been born in 1964.
In a section of Behind the Laughter, Lisa said that to prolong the series, she was forced to take anti-growth hormones, however, this episode is non-canon.