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.
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, Marge 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 two years (in the Simpsons-verse) to remodel, but no-one has aged.
Real Time Spread Out
One way around the timing issue is to suggest that the Simpson characters are modeled off the actual Simpson family and that the events being presented are the happenings of the real Simpson family over a period of just under two years, with future forecasts and past recollections being reenactments. Hence a kitchen remodel which takes two years could reasonably refer to the latter portion of one year which then flows into the second year. This theory allows for there to have been enough time in the two years of the real Simpson family for there to have been over 600 different thirty minute episodes, keeping in mind that several episodes and substories could all be happening on the same days with overlapping times.
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.