Most shows that stay on the air for more than a decade experience some sort of change. A key cast member leaves, as on “Law & Order.” Kids grow up, as on “Seventh Heaven,” or the show simply seems to lose its focus, as on “ER.”

Not so “The Simpsons.” On Fox’s animated show, things have stayed the same for 18 years: Homer’s still an idiot, Marge is a martyr, and the residents of Springfield are deviantly dysfunctional.

That constancy explains a good part of the show's success through the years. With so little change, and little in the way of back stories, it's all the much easier for kids to jump into "The Simpsons." It doesn't demand the attention spans of shows with long, winding storylines.

In that regard it's a classic cartoon along the lines of "Tom & Jerry” or “SpongeBob SquarePants,” completely self-contained, though much more sophisticated.

Once in as teens, viewers may remain with the show for years after.

That pattern is evident again this year. Sunday’s season premiere episode of the long-running show averaged a 5.7 rating among teens 12-17, up 14 percent over last year’s 5.0 debut.

That’s despite the fact that longer term, among adults 18-49, the show has seen its audience erode some 30 percent over the past decade. Among households and 18-49s, its premiere was about even with 2005, with the show still airing in the same 8 p.m. timeslot and with the same NFL post-game show lead-in.

This year, in a sure boost to ratings, Fox has been aggressively promoting "The Simpsons" through new media venues popular with teens. A week ago the network, which is owned by News Corp., made seven minutes of the “Simpsons” premiere available on social networking site MySpace, which is also a News Corp. property and had some 46 million unique visitors in July.

News Corp. president Peter Chernin said earlier this week that original “Simpsons” episodes will be produced for mobile phones before year’s end, tying in with the company’s acquisition of mobile video producer Jamba.

Meanwhile, in broadcast and basic cable ratings for younger viewers for the week ended Sept. 10:

Among teens 12-17: Fox was first at 2.3, trailed by NBC in second with a 1.6, ABC third at 1.4, Univision fourth at 1.0, CBS at 0.9, the WB at 0.6, UPN at 0.3, Telemundo at 0.2 and Telefutura at 0.1.

Among kids 2-11: Fox took first at 1.4, with NBC and ABC tied for second at 1.0, followed by Univision at 0.9, CBS at 0.7, the WB at 0.4, UPN at 0.3, Telemundo at 0.2, and Telefutura at 0.1.

On basic cable for the week ended Sep. 10:

Top five shows among 2-11s: 1. "FOP Movie Channel Chasers" (Nickelodeon); 2. "SpongeBob SquarePants" (Nickelodeon); 3. "Fairly Odd Parents" (Nickelodeon); 4. "SpongBob SquarePants" (Nickelodeon); 5. "Fairly Odd Parents" (Nickelodeon)

Top five shows among 6-11s: 1. "Jimmy Neutron" (Nickelodeon); 2. "FOP Movie Channel Chasers" (Nickelodeon); 3. "Fairly Odd Parents" (Nickelodeon); 4. "Fairly Odd Parents" (Nickelodeon); 5. "SpongeBob SquarePants" (Nickelodeon)

Top five shows among 12-17s: 1. "Flavor of Love 2" (VH1); 2. "WWE Entertainment" (USA); 3. "Laguna Beach" (MTV); 4. "Two A Days" (MTV); 5. "WWE Entertainment" (USA)

Top five shows among 12-24s: 1. "Flavor of Love 2" (VH1); 2. "RW/RR Fresh Meat" (MTV); 3. "Monday Night Football" (ESPN); 4. "Laguna Beach" (MTV); 5. "Family Guy" (ADSM)