Big Bird is still 6 years old. But the original "Sesame Street" kids — that first generation of preschoolers who first met Cookie Monster and Oscar the Grouch and sang along with "Rubber Duckie" — are in their (gulp) 40s now.

And, as of Tuesday, so is "Sesame Street," which made its debut Nov. 10, 1969, when the haze of Woodstock still hung in the air, the Vietnam War raged and Neil Armstrong had just taken that giant leap on the moon.

PBS president Paula Karger says that when she mentions the anniversary, some people smile. Others look stricken because they can't believe so much time has passed. But plenty of "Sesame Street" kids are parents or even grandparents who have spent quality time on the couch with new generations of little fans.

Luckily, grown-ups have always been able to enjoy "Sesame Street" almost as much as children, appreciating humor that younger viewers don't get yet.

"That's always been one of the secrets," says Michael Davis, whose book "Street Gang," a riveting look at how "Sesame Street" came to be, is just out in paperback (Penguin, $16).

"The second level, the wink to parents, is deliberate, because Joan Ganz Cooney and the other creators of the show were TV pros who wanted to make a show for kids that wouldn't make them retch."

The seed of "Sesame Street" began to sprout in 1966, when Cooney gave a dinner party at her New York apartment and invited Lloyd Morrisett, a vice president of the Carnegie Foundation.

Morrisett remarked that his daughter Sarah, 3, was so obsessed with television that she would get up early and quietly sit in front of the TV set on weekend mornings, watching the test pattern and waiting for cartoons to come on.

"It was the same thing millions of kids were doing all across the country, an image that confounded Cooney," Davis writes.

Before long, she was brainstorming with Morrisett and others on "how to master the addictive qualities of television and do something good with them."

"What if," she wondered. "What if you could create content for television that was both entertaining and instructive? What if it went down more like ice cream than spinach? What if we stopped complaining about the banality we are allowing our children to see and did something about it?"

In perhaps the pivotal moment of "Sesame Street" creation, Cooney and company recruited Jim Henson, whose Muppets had previously starred in a series of coffee commercials in the Northeast. Scenes featuring human characters on a realistic street had tested poorly with kids, whose attention wandered. But once the Muppets and humans began interacting, children were captivated.

Beyond all the fun, "Sesame Street" was born as a daily show for preschoolers that was "inoculated with a stealth curriculum," Davis, a former TV Guide columnist, said Tuesday from his home in Pennsylvania.

"It wasn't the first TV show to teach children, 'Captain Kangaroo' did it well," Davis says. "But 'Sesame Street' brought the curriculum, created by educators and researched and tested to help children learn."

To this day, "Sesame Street" produces a new preschool curriculum every year, says Rosemarie Truglio, vice president of education and research for Sesame Workshop (created by Cooney as the Children's Television Workshop).

Kids might not know they're learning, and they might not recognize some of the guests who turn up regularly. But everybody who's anybody, it seems, has visited "Sesame Street."

Astronauts and actors drop by. So do politicians and professional athletes, plus musicians of every stripe, from Chubby Checker to Yo Yo Ma. Andrea Bocelli once crooned a goodnight song to Elmo.

When Michelle Obama appears on Tuesday's anniversary show, she'll follow previous first ladies Barbara Bush, Laura Bush and Hillary Clinton, plus everyone from poet Maya Angelou and United Nations Secretary General Kofi Annan to Jim Carrey, Robert De Niro and Sarah Jessica Parker.

"Aside from 'The Tonight Show' and maybe 'Ed Sullivan,' no show has had this array of guest stars," Davis says. "In the beginning, the producers just called up people they wanted to see on the show, and some accepted. But then stars started calling them, and they still do — especially actors who want to be big in the eyes of their children, because their kids might not have been able to see some of their other work."

Watch early "Sesame Street" (many clips are available on YouTube) and a grown-up fan may marvel at how different the show seems today. Some complain there's been too much change.

Children have changed since 1969, and so has parenting, Davis says.

"The show has been in a constant state of evolution, testing and checking itself," he says. "It never stopped growing and trying new things, and that's a big part of the answer to why it's still on the air."

"Sesame Street" producers talk a lot about change when discussing the 40th anniversary season.

In fact, so much is new this year that "Sesame Street" boasts that the new season will be brought to you by the letter N — "new show format, new show open, new nature curriculum and new Abby Cadabby CGI animation." (Abby, an adorable 3-year-old fairy-in-training introduced to increase the female presence among the characters, quickly became a popular favorite.)

After four decades of separate segments, magazine style, the new format calls for an integrated block with a host, Murray the Muppet.

"When we premiered in 1969, the goal was to use the most up-to-date media, most attractive to kids, to help them learn and reach their full potential," says Miranda Barry, Sesame Workshop's executive vice president for content. She and Truglio talked about the anniversary with TV critics this summer in Los Angeles.

"In 1969, the hit show was 'Rowan & Martin's Laugh-In,'" Barry says. "But now, kids are more accustomed to seeing longer-form programming, full stories."

Murray the Muppet will take viewers through a predictable pattern each day, Barry says:

"The Street Story, which is all our classic stories; the new Abby Cadabby piece; a third act that's a surprise that allows us to have an experimental corner within the show; and, of course, Elmo's World."

"Sesame Street" had resisted using computer-generated imagery.

"When you're dealing with something that has this kind of history, you don't want to do things that are so radical that people say, 'Oh, that doesn't belong,'" Barry says.

But the new season will feature its first CGI segment, "Abby's Flying Fairy School."

"It did take a while for us to get our heads around the idea of trying an animated version of our puppets," Barry says. ":But on the other hand, we want to be the most attractive thing to kids.

"The school itself flies, which is pretty cool," Barry says, but the characters, which are three-dimensional, can visit the street, "so I think they fit in really well with our puppet characters."

Truglio says that each new curriculum focuses on critical needs for the current crop of children, including literacy, the focus for the past two years. Next up: science, "through the lens of nature," that will encourage children's innate sense of wonder about the natural world around them, even in their own neighborhoods.

Parents of preschoolers of 1969 had few television choices. Today, they have dozens, on an array of cable networks dedicated to children.

"In a way, this is the golden age of preschool television," Davis says. "There are so many quality programs, many with their own curriculums. But all roads lead back to 'Sesame Street.' That's the one that inspired it all."