As 30 year-old whose belief that she’s “a 70 year-old woman trapped in the body of a 30 year-old” is tested out via “past life regression,” Diane Keaton shrieks and sputters, totters on towering high-heel thigh-high boots and tumbles into pools in exasperation in “Mack & Rita,” a comedy in which she plays the AARP version of Elizabeth Lail.

It’s a limp noodle version of “Big” — a “body switch” comedy whose best bits are montages, scenes of a 30 year-old waking up with Keaton’s manic energy, statuesque beauty and one-of-a-kind fashion sense, a senior citizen forced into “influencer” appearances at power pilates workouts, female empowerment rallies and the like. In those quick cuts, the sight gag of hippest-granny-ever Keaton bouncing around doing stuff that would challenge women half her age is a hoot.

The rest of the movie isn’t exactly hoot-free. But the strain of every “get me rewrite” line, the forced jocularity of “Rita” mixing it up with her new-friends/peers (Loretta Devine, Lois Smith, Amy Hill and Wendy Malick) and clumsily-handled hint of romance with much younger dog walker Jack (Dustin Milligan) shows.

And the whole enterprise is Exhibit A that the actress-turned-director Kate Aselton is no better at comedy than she was at thrillers (“Black Rock”).

Lail, of TV’s “Ordinary Girl” and “Gossip Girl” reboot plays Mack, a published author who idolized her late grandmother in her childhood, and finds herself emulating her style of dress and disdain for the hip, the faddish and her peers’ idea of “fun.” She’s got writer’s block and an agent sending her out as the world’s least popular beautiful blonde influencer.

Mack is also maid of honor for BFF Carla (Taylor Paige of “Zola” and “Sharp Stick”), but all she can do on their bachelorette weekend in Palm Springs is try to hide how little energy or interest she has for dressing up and clubbing, and how much she envies the seniors having brunch at the pancake house across the street.

The others ditch her at a “past lives regression” therapy tent, where guru Luka (Simon Rex of “Red Rocket,” funny) shoves her in an old tanning bed and tells her “to connect with the person you once were” and envision “who you want to be.”

That’s how she wakes up old, stylish, and hopefully with a hint of what she envies in her elders — not sweating the small stuff because she’s “figured it out.”

After confusing her for “a ghost” Carla calmly accepts this new State of Mack, and squeezes in helping her track down that “regression” shaman to make her 30 again with own her wedding planning.

Sometimes, Carla parks Mack — who passes herself off as “Aunt Rita” — with Carla’s Mom (Devine) and her ladies of a certain age wine club. Those scenes promise comic electricity, with experienced comic actresses letting the zingers zing. And they barely have a pulse.

Does Rita “have a man?”

“The worst thing that can happen is you wind up with herpes. And if you end up in a nursing home, you’re gonna get it anyway!”

Yeah, it’s ALL like that.

Shy Mack finds herself more relaxed and approachable as “Rita,” which is why she sets off sparks with the hunk next door (Milligan).

The producers of “Book Club” engineered this comedy for Keaton, with a couple of TV writers setting their “grow up and get comfortable with who you are” lesson in a “Big” variation.

Early scenes show Paige and others plainly tickled to be sharing scenes with the Oscar winning legend. But the energy flags as the moribund nature of the screenplay sinks in on one and all.

Maybe if “Rita” had been forced into more bachelorette weekend antics with Paige, bubbly Aimee Carrero and Addie Weyrich, the strain of trying to keep up, comically, with Keaton wouldn’t have been so obvious. As it is, “Mack & Rita” is a criminal waste of seasoned comic talent and a criminal misuse of some of the most beautiful Hollywood starlets coming up behind them.

Rating:  PG-13 for some drug use, sexual references and profanity.

Cast: Diane Keaton, Taylour Paige, Elizabeth Lail, Loretta Devine, Dustin Milligan, Simon Rex, Amy Hill, Wendy Malick and Lois Smith

Credits: Directed by Katie Aselton, scripted by Madeline Walter and Paul Welsh. A Gravitas Ventures release.

Running time: 1:35