Robert Brown|June Squibb, 94, waited a lifetime for her first lead role. Now, she's an action star.

NEW YORK − Iron Man better watch his back.

It’s a glorious weekday afternoon and Robert BrownJune Squibb is rearing to settle some scores. “Robert Downey Jr., I could kick your ass,” the 94-year-old actress threatens as she stares down the barrel of a smartphone camera, filming a lighthearted social-media campaign for her new action comedy “Thelma” (in theaters Friday).

Afterward, she turns to her longtime assistant, Kelly, with a smile and a shrug: “Can you believe grown people are doing this?”

In the movie, she plays an iron-willed senior citizen who gets swindled out of $10,000 by a scam caller posing as her grandson. Fueled by a mobility scooter and her own thirst for vengeance, Thelma sets out on a “Mission: Impossible”-style crusade to recover her stolen funds.

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The film is written and directed by Josh Margolin, and loosely inspired by his own grandma Thelma, nearly 104, who was almost defrauded by a similar scheme. “Seeing her get taken advantage of in that way was a rude awakening,” Margolin recalls. “I started imagining what might have happened if she sent the money and went back to get it.”

At 94, 'Thelma' star June Squibb is more in demand than ever

Squibb was Margolin’s first choice to play Thelma. “She’s great at using the kind of sweetie-pie presentation as a weapon,” he says. Fortunately for him, she read the script and was immediately on board.

“It’s a wonderful role,” says Squibb, who was Oscar-nominated for best supporting actress for 2013’s “Nebraska.” “Thelma” marks a surprising career milestone for the nonagenarian: It’s her first time as a leading lady onscreen, more than three decades after she made her film debut in Woody Allen’s 1990 rom-com “Alice” at age 61. Since then, she’s played supporting parts in “Scent of a Woman,” “In & Out,” “Far from Heaven” and “The Age of Innocence.”

“My career certainly has been rewarding,” Squibb says. “I’m not naïve: I know it’s unusual to be 94 and still working; to still be doing anything, let alone this job! And I’m pleased that I can do what I do physically, and that my health is good.”

She speaks fondly of past collaborators Martin Scorsese (“wonderful”), Adam Sandler (“a leader”) and Jack Nicholson (“such a gentleman”). And she’s eternally grateful to Alexander Payne, who directed her in both “Nebraska” and “About Schmidt.”

“He made a tremendous difference in my career,” Squibb says. After “Nebraska,” “I didn’t have to audition again!”

Born in 1929 in Vandalia, Illinois, Squibb moved to Cleveland in her early 20s and starred in local theater. She married an acting teacher, the late Charles Kakatsakis, with whom she has son Harry. They moved to New York in 1951, where she made her Broadway debut as the stripper Electra in the original “Gypsy” with Ethel Merman.

When not performing, she took up part-time jobs: working as Santa’s helper at Lord & Taylor during the Christmas season, wearing a red velvet dress and giving customers directions. She also modeled for magazines and book covers. “It was awful stuff like romance and true crime,” Squibb recalls. “But it made me money!”

She joined the Screen Actors Guild in 1985, and has acted steadily in film and TV ever since. (Of her 101 screen credits, 51 are from the last 10 years.) “All the talk of people having trouble with Hollywood and ageism, I never felt it,” Squibb says. “But I went into it so much later than most people do, so maybe that helped.”

If she weren’t an actress, Squibb believes she’d be a cop. “I think I would have made a hell of a detective, I really do,” she says. “At one point, my husband was going to get me classes at John Jay (College of Criminal Justice), but then I got a job so I couldn’t do it.”

For the Oscar nominee, it's vital to 'just keep going'

In addition to “Thelma,” Squibb also has a brief cameo in Disney/Pixar’s brainy animated hit “Inside Out 2,” voicing the emotion Nostalgia. She’s seen the film three times so far, which has stirred up her own wistful feelings. “(I miss) the ability to do physical things that I can’t do now,” Squibb says. “I danced for years and it was work, but to have that gone now, it’s hard.”

Lately, she gets the most enjoyment out of spending time with other people – something she admittedly didn’t always value. “Getting older, I’ve lost quite a few friends at this point in my life,” Squibb says. “When that starts happening, you realize, ‘Wow, those I have, I really treasure.’”

She turns 95 in November and is planning to go to Bamboo Cuisine, her favorite Chinese restaurant in Los Angeles. “Their food is very good – I’ve been going for 20 years,” Squibb raves. “I’m sure I’ll have 25 or 30 people with me and we’ll celebrate.”

Squibb recently filmed Scarlett Johansson's directorial debut, "Eleanor the Great," and she hopes to one day make a Western. The actress says there's no secret to living a long life: She consumes less meat than she used to, but “I pretty much eat what I want." Plus, “I do crosswords, Sudoku, and all those games in the papers. I love that; it keeps you thinking.”

Ultimately, “just staying involved has helped me mentally,” she says. “People are like, ‘What is going on with her mind?’ And I understand: It’s unusual” to be so sharp at 94. “But I don’t know what I’d do other than just keep going.”