Friends Jane Fonda and Lily Tomlin don’t just make each other laugh on Grace and Frankie. They crack each other up in real life, too. “Once, I had to say the line, ‘Do you want me to blow some of my courage into your mouth?’ and we couldn’t stop laughing,” Lily recalled at a recent PaleyFest event for the show in Los Angeles. Adds Jane, “It went on for 30 minutes — the crew was furious at us!”

Still, lighthearted moments like these keep Jane, 81, and Lily, 79, youthful. “I wrote a book about aging, and I interviewed a dozen people who were over 100,” Jane says. “All of them had a sense of humor.” Agrees Lily, “You just need to be playful.”

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The good feelings on the set of Netflix’s hit sitcom have proven infectious with viewers. “We didn’t expect young people would like it as much as they seem to,” says Jane of the show, which has been picked up for a sixth season. “Grandparents, parents and grandchildren can all watch it together, and it works.” The series’ cross-generational appeal, says Lily, “shows you that older people are just as vibrant, complicated and worthy as any other age group.”

Jane and Lily have been close ever since they costarred with Dolly Parton in 1980’s classic comedy 9 to 5. All three are on board to appear in a planned big-screen sequel. “Everyone is very hopeful,” Lily says of the film, which is currently being written. “The script is a hot property, so I think we may not live to see it!”

Another comedic highlight from Lily’s past, the ’60s variety show Laugh-In, recently held a cast reunion in LA that was filmed and will be released on Netflix. “It was great,” Lily says. “It was really surprising because the jokes seemed so current, and they were 50 years old.”

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Lily will celebrate another milestone — her 80th birthday — in September, and she’s already got big plans. “I’m going to borrow some money from Jane,” she jokes. “And I’m going to fly a bunch of my friends to either Morocco or Mykonos. I think Morocco’s more exotic.”

With age comes wisdom, of course, and Lily and Jane have plenty to share. “You learn what you actually need and what you can let go,” says Jane. “You let go of a lot of things and everything becomes much lighter.”

And, as Lily puts it, “If you stub your toe on the bed stand, you learn to avoid doing that. Or if you hit your funny bone, you go, ‘I’m not going to do that again.’” That may be true, but we hope these two never stop hitting our funny bones!

For more on Jane Fonda and Lily Tomlin, pick up the latest issue of Closer Weekly, on newsstands now — and be sure to sign up for our newsletter!