brakusaetsya
My two cents - it's just hard to picture a teenager's life so another teenager would see and like it, that's why there are no teenagers in the Simpsons. Same story with Futurama, most of the characters don't have ages at all (how old is Bender? Several thousand years iirc) Leela is more a mature woman than a teenager, and other characters except Fry cannot be called teens as well.
On another hand, that Simpsons episode with university nerds was great, I'd say it was one of the funniest episodes ("HOMER: Hey, that's not a wallet inspector at all..." ). So probably both Simpsons and Futurama are missing something, ignoring certain ages in their characters.

Trivia: Bender's Head is 4, his body is 1000-odd years old
More Trivia: Bart fell in love with Darsey (Little Big Girl) who was 15, (DARSEY'S DAD: He took advantage of a girl who was 5 years older than him!) Bart also almost married Darsey because she was pregnant.

I don't think that the fact that there are no teenagers in the Simpsons Family is something intended. It's just a coincidence, Groening had to choose something when he was creating his characters, and he decided to stay on usual "parents-kids" version. In other cartoons, different versions are possible. I don't think the absence of teens amongst primary characters in the Simpsons is a loss, and that's why: you can make a kid act like a teen (we've seen this many times, when Bart, or Lisa acted more mature than they are) - but you can't made a teen act like a kid, it would look stupid.

ALM
Really? I thought it's vice versa.

brakusaetsya
Maybe there is a sense in your words, the Simpsons-world is more familiar and "domestic" to us and maybe because of this very reason we love it so. I can remember clear when in 2005 I was respected Futurama much more than so usual and boring serial. When I started to watch the Simpsons regularly since 11...12 season (somewhat 1999 I think) those series doesn't have so much influence to me. The fire was flamed up after watching "centralizing" the episodes from 1...5 seasons I've purchased in february 2006 during my university vacations.
But and Futurama has possessed me quite much as I've mentioned. I can't speak of any obvious reasons why I like Simpsons more than that. Maybe it's only my fear to lose so long-gathered adoration to yellow family after some sessions of watching the "competitors", because I remember how I liked that, especially the episodes with machine "What if?.." (if I not missed with a name)

So, I think now it's time for me to write my second post.

Yes, in Simpsons more attention is  paid to children and adults. So, there aren’t many teen characters in Cartoon. But is this so surprisingly? We observe in the whole serial the life of a little American town, witch is the second after the most bed town in the world. Do you want to live in such place (I don’t ask, would you like to live in Springfield)? I think no… as well as the most people. So, who lives in Springfield? – People, who can’t leave (or don’t want to change their life) and their little children, because they can’t live alone.

Second reason: The main characters are two parents and two children about 8-10 years old. (I don’t think that Maggie is a main character).  It can be very hard - to connect these age categories with teenage category. – The Simpsons’ family can go on the second plan. Do you want this? Without teenager life, we can see two subject lines. First is a line of live “adult town” and a second – children at school. And these lines are connected together in family of Simpsons. So, we can see life of a typical family and life of the whole town. (The life of a town is the life of FAMILIES, living in it). Teenagers are something an average, and their life is absolutely other history.

In the “Futurama” authors didn’t put the purpose to show a typical life. And in this cartoon every character is an individual. In this position it more easily to show large of universal. =) But there aren’t identical people – there aren’t standard society.