Umm, Bart just did time in juvie: S15E16 “The Wandering Juvie”. Not a memorable episode (e.g., it had stunt casting), so I can understand the lapse.
I didn’t see that episode, and the episode guide summary I read didn’t make it look appetizing.
But again, my point is that Bart in that episode was a 10-year-old shackled to a girl in imitation of The Defiant Ones. Imagine two 16-year-olds in that situation.
Hey, what you’re doing is disgusting. Stop imagining it.
See the difference? 
Stunt casting?
[QUOTE=Marley23]
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. QUOTE]
mr burns was over 1000 once, when they asked him to key in his age it was a joke that he pressed 4 buttons to do it.
Casting a celebrity voice for the sake of casting a celebrity. In this instance, casting Sarah Michelle Gellar as the voice of the girl juvie escapee.
…You just noticed they do that? They have a guest star almost every episode these days. It’s not really even done to draw fans in at this point, it’s just because they can. I don’t think casting her really made it any different from using a different celeb or a member of the regular voice cast.
Some day they will age them, I believe. I can see them doing what they did with the Rugrats: when the series seems old and tired (ok, I thought it did after season 6, but what do I know?) they’ll make a Simpsons spin-off with the characters older. The Simpsons: All Growed Up. You watch, it’ll come 
The thing is, that won’t give them much new material. If they want to do jokes about working in the adult world, they already have plenty of adult characters to work with. The novelty of it being Bart and Lisa instead of Homer and Marge* would probably get old after an episode or two, unless the writers are really clever.
*No, I don’t mean Bart and Lisa are married to each other.
the other downside to having Bart and Lisa grow up and ‘discover sex’ is that it would almost have to be crass and crude in a way the Simpsons has rarely been. (I know I’ve forgotton some crassness and crudeness, but work with me here).
There’s plenty of other shows that have handled the ‘wackiness’ of characters discovering or being sexy. Hell, even Homer and Marge have been sexy ([homer] ‘can’t get image of stupid sexy Flanders out of mind’[/homer]), but for Bart and Lisa… well, they already have discovered sexuality already and it is coy and amusing (Bart working at the bordello, Bart selling tickets to the cable porn channel -‘Top Hat Entertainment’). Going that step further (with an older Bart or Lisa) would be nasty and cheapen the show. Wouldn’t it?
And as has been said, everything that needs to be done can be handled with the current framework, why mess with it?
Some day they will age them, I believe. I can see them doing what they did with the Rugrats: when the series seems old and tired (ok, I thought it did after season 6, but what do I know?) they’ll make a Simpsons spin-off with the characters older. The Simpsons: All Growed Up. You watch, it’ll come
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Spinning off a show that’s been on the air for 16 years (or more) sounds pretty stupid to me. I think Yumblie is right on.
Not that stupid, as long as the new premise is decent enough to hold things afloat. Cheers ran for eleven seasons, and then spun off to Frasier, now finishing up after eleven of its own. I’m blanking on other examples right now, but it seems reasonable to say it could work. Just need to have Bart and Lisa move to Seattle to become hoity-toity psychologists. Or something.
They don’t age because of the wonders of modern science.
Professor Frink installed one of his patented “Age Stablizinators” in the Simpson’s basement.
“What with the formaldehyde, and the goo, and age-defying complex, and the glaven… and the…Hey! Lady!”
Futile gesture has it right. It’s not just cartoon characters that don’t age – hardly any sitcom character ever ages.
Look at Jay North in Dennis the Meance – in later episodes he was almost as tall as his mother, but he played the same character in the same way. When Ritchie on the Dick Van Dyke Show got to big to be a cute little kid anymore, they gradually eliminated his character.
Even when the kid is allowed to grow up (Opie on the Andy Griffin Show, Beaver and Wally Cleaver) the parents don’t change. Lucy and Desi had a baby, saw it grow up, but never changed themselves. Any kid who had the Ricardos as parents and the Mertzes as his only other adult role models should have been put in foster care.