Springfield is stuck in a Time Warp.
Maggie and Bart are always in grade school because the whole town of Springfield is stuck in a 'space-warping tesseract.'
And you thought it was because it was a cartoon.
I was 10 years old when The Simpsons began its history-making run in 1989.
Now I'm aging and raising a family and stuff, while Bart and Lisa are still stuck in grade school.
What gives?
I have always attributed their static state to an "if it ain't broke, don't fix it" mentality.
After all, the show is still generating laughs more than two-and-a-half decades later, so clearly the lives of a "young" family provide plenty of fodder for a successful comedy.
Some fan theorists, however, insist that the city of Springfield is actually "trapped inside a space-warping tesseract" [source: Vitto].
The rest of the world can change and grow all it wants, but the residents of Springfield are simply not able to, because they are trapped in a time loop.
The characters do recognize that time is passing (for instance, the family visits Italy where Sideshow Bob is now living with a wife and child), although they remain unaffected by it.
The theory also accounts for how the characters are able to take the occasional trip, since tesseracts can shift location as needed.
It's just as well for everyone to maintain the status quo, anyway.
I'm pretty sure Maggie would snap if someone took away her binky after all these years.