I asked myself the question several times. I am quite fond of the (usually futuristic) episodes which deal with grown-up Bart/Lisa/Maggie and aged grown-ups.
To me it seems, that several writers yearn for such a development as I am under the impression that there is a growing tendency to depict Bart, Lisa and their peers in situations / behaviours which neither suit 8- nor 10-year olds.
Having main characters (the ones we know as well as alternate offspring of them) go through every age sounds like madness on the one hand, but it would of course give the writers more room for ideas. It could portray the characters consistently as representatives of their generations. It would also give them the possibility to create "event-episodes" - marriages, births, deaths of main characters. (I know I start to sound like Cristof from "The Truman Show")
Actually, to me, no cartoon series come to mind either which depicts their characters aging (I think even the one or the other live-action series has its issues keeping track with their aging flesh-and-bone-thespians). I wonder, however, if someone had told Matt Groening from the outset, that "The Simpsons" would last for more than three decades, if he had opted for this totally unusual way. What the Simpsons actually did from early on (and which is unusual, too) is that they let characters die "on screen" and for good. Bleeding-Gum Murphy was probably the first, and Flanders being widowed TWICE is the most extreme example.
Now with the Simpsons (as a series) starting in 1989 that means Bart was born in 1979, Lisa in 1981 (both rather GenX) and Maggie in 1988 (opposite to their older siblings a typical Millenial); Homer and Marge were probably about 34 in the early seasons (their ages have never been consistent and they tend to be depicted as closer to 40 by now, perhaps reflecting later marriages and births in Western society); that makes their birthyear ca. 1955- typical baby boomers.
Abe would probably have been born in the 1910s, representing the "Greatest Generation" (he is always depicted as an "old dad" even when Homer was young and fought in WW2). As often depicted, Mr Burns would probably be roughtly of the same age. While we would have witnessed Abe's death somewhen around 2000 slightly above average life expectancy; Mr Burns might still be around with his treatments to actually become the oldest living human being a running gag.
Now, in 2023, Homer and Marge would have their 68th birthdays (and despite his habits, I am quite certain that Homer wouldn't be written off the show that young - unhealthy lifestyle makes an early death more probable, but not certain); Bart would become 44 this year (spoiler: that makes him a year younger than me), Lisa 42 and Maggie 35.
This means currently most characters representing a profession would have passed the torch to a character we don't know or someone who is forever stuck in Elementary School in our world. (Imagine Ralph Wiggum turning out to be more competent than his father due to his childish moral compass.)
This also means a new generation of Simpsons. Barts oldest offspring would most probably be in their early 20s by now. (Bart might even become a grandfather soon.) Lisa's child/ren will probably be significantly younger, she might also be a single mom. With the little we know about Maggie, she is a blank slate and improbably to predict. She might be written into an LGBTIQ-character (and nevertheless by now raise one or more children).