I know it’s an old convention of comic strips and TV cartoon series that the characters can remain the same age indefinitely. But it’s not required (see Gasoline Alley.) And with The Simpsons, what a bet they’re missing! Think what they could do with Bart and Lisa hitting puberty and discovering sex!
Nah. Why should they? They just have to have a “flash-forward” episode to show the group older. They have done this numerous times: The vision Bart sees at the Indian Casino, the gypsy fortune-teller that shows Lisa her English beau in college, etc. There is really nothing to gain by letting them age, and many possible snags to storylines.
If they make Bart and Lisa teenagers, they run the risk of the horrible plague that befalls many sitcoms, the “very special” episodes. Bart and Lisa already act a lot like teenagers anyway (Lisa being mentally beyond her years and Bart having a pretty extensive vocabulary and knowledge of pop-culture for a 10 year-old), so there’s plenty of opportunity for jokes like that.
Plus, they’ve already done a couple “love” plots, which of course can’t lead anywhere because everything has to be the way it was by the end of the episode. What better way to do that then have the kids be young, so any “relationships” that happen won’t really mean anything and can easily be dissolved by the episode’s end.
Besides, if they kids got too old they’d have to introduce Cousin Oliver.
And we all know what happens then…
If nothing else, because it would be weird if they started now. Just like it would’ve been weird if they’d started aging after 10 years or 5. It’s a sitcom, and in sitcoms thing don’t change too much. Since it’s animated, they don’t even have to deal with things like child actors aging. And I think that Yumblie is right in that Bart and Lisa don’t act their ages most of the time anyway. Nor does Homer, come to think of it. 
South Park is in it’s 8th season and the boys have only advanced from 3rd to 4th grade. The formula works, the situation works, the movie worked, why mess with it? As said before, regular sitcoms have to deal with child actors growing up. That forces different story lines to coincide with “real” situations they would face.
Leaving Lisa at 12 (or whatever she is) is a lot easier to write for than the Olson twins. Writing for the twins was different when they were 4 than when they were 8. It’s just easier to stick with what works.
What’s funny is that this season there was an episode that included Apu and Majula’s children, and they had aged, possibly even in real time. They were slightly older than Maggie.
She’s 8…that’s really all I have to add to this discussion.
Sorry…I’m a Simpsons fiend.
What amuses me is that Bart and Lisa have graduated least twice each, and they’ve had a birthday or two (and so has Maggie, I think), but they remain the same age. Stuff like continuity is a lot less important to the writers than being funny is.
Another complication would be having to blow off a good percentage of their great vocal talent.
Bart, Milhouse, Rod & Todd, Ralph Wiggum, Martin, Nelson, Jimbo, Dolph, and Kearney are all voiced by Nancy Cartright, Pamela Hayden, Russi Taylor, and Tress McNell.
It makes sense to have juvenile boys voiced by adult women. It would not make sense to ask the same adult women to modify the characterizations they’ve perfected by attempting to age them. It would make even less sense to let this great talent pool fall by the wayside and come up with all-new adolescent and adult voices for these characters.
Would you beleive I almost wrote “8” but thought I had it confused with the South Park boys? I don’t watch Simpsons enough.
A vital rule of any good sitcom (which is what The Simpsons is essentially) is: Nothing ever changes. Every episode’s end should return the characters to exactly the same point they started from. Otherwise you eventually destroy the situation that established the balance of the show in the first place.
This is why any family-based sitcom eventually ends up a shadow of its former self. (And many have ended up sucking terribly, Rosanne anyone?.) You can’t stop everyone aging, the kids have got to grow up, change is inevitable. The Simpsons are in the lucky position that this is not a problem. So why should they ever want to change a set-up that works so well?
Obviously if nothing ever changes you eventually you reach the point where all plots and ideas are exhausted. That’s the point to call it quits rather than trying desperate changes. The Simpsons, unfortunately, are periously close to this.
One character is aging – Mr. Burns. In the first episodes, I believe he was in his 90’s. Now he past the century mark. Last episode I saw that mentioned his age, it was given as “107.”
They would also miss a lot of self-referential humour.
From the power bar episode: “Maggie doesn’t seem to be growing at all.”
Now wouldn’t it be funny if they went to college while Lisa was still 8?
Anything to avoid any Very Special Eps about Bart and/or Lisa getting married and having kids. I know the writers would have fun with it, but … yech!
The good old SNPP says that the first time his age was mentioned (Simpson and Delilah), he was 81. It’s not that he’s aged so much as they decided the older they made him, the funnier it would be. Homer’s age hasn’t always been the same either, they say it’s been given as 36, 38 or 39. They picked 39 and stuck with it later. I think part of the issue is that early on, nobody was paying attention to that kind of stuff, so there were contradictions. Later on, I think the writers started deliberately contradicting things everybody “knew” for laughs or to annoy picky fans.
With each other?
I have nothing to add other than to point out that the Southpark boys are also 8 
Well, I did see a cartoon on the 'net once.
Homer: I ain’t paying for 2 weddings.
I believe that was the episode where they were living in a trailer in the deep south for awhile.
Also remember that when Bart pulls pranks as a 10-year-old, it’s cute. If he pulled similar stuff as a 16-year-old, it’s thuggery. Not that I would mind seeing Bart in juvie, but most people wouldn’t.