When the trailer for Friday’s “Rambo: Last Blood” dropped, doubters predictably began sharpening their knives for its now-73-year-old star.

What was the plot for this fifth installment, they wondered on social media? Sylvester Stallone yells at clouds and tells youngsters to get off his lawn?

Comedian Alice Taylor-Matthews suggested the movie’s tag line should be “Revenge is a dish best served old.”

“Go eat some pudding!” another hater ordered Stallone.

The joke, however, might be on them. As easy as it is to make cracks about a star’s age, the current “geri-action” boom that has been Rascal-ing along for a few years shows little sign of abating.

The reality is Sly, Arnold Schwarzenegger (72), Dolph Lundgren (61) and other members of the action A-list continue to get work long after the Josh Hartnetts and Taylor Lautners of Hollywood have been blown away by a stiff breeze.

“I think they’re icons,” producer Natalie Burn tells The Post. Her film “Acceleration,” starring Lundgren is out in November. “There are not that many actors today that are as good as these guys, so we’re still going back to the classics,” Burn says. “Audiences are missing those characters that they grew up on.”

Bruce Willis is now 64, Clint Eastwood is 89, Denzel Washington is 64 and Linda Hamilton, who is returning to the “Terminator” franchise with November’s installment, “Dark Fate,” is 62.

Researcher Stephen Follows found that the average age of action stars started creeping up in 2015 to near 50 from 40 in the previous two decades.

Writer-director and New Jersey native Lior Geller, whose recent film “We Die Young” starred Jean-Claude Van Damme (58), says age often serves the actors well.

“Usually the roles that they play now are a little deeper,” he tells The Post. “These actors bring a certain life experience to these roles that sometimes younger actors can’t portray in the same way.”

In other words, grizzled can be good.

Not that these actors are out of shape. Hamilton’s arms in “Dark Fate” look like they time-traveled from 1984, and you might be able to get a wine stain out of a shirt on Stallone’s abs.

“There are a lot of 30-year-olds going in for auditions who wish they looked like Sly today,” Gunnar Peterson, Stallone’s trainer, tells The Post. “I’ve joked with him for years that he’s like ‘The Picture of Dorian Gray.’ He’s defying the odds.”

Stallone works out at least four days a week for an hour, lifting weights, doing push-ups and boxing.

And while what’s under their shirts may help bring in an audience, what’s above the title may count for more. Their names still carry huge weight, especially overseas.

Stallone, Arnie, Lundgren and the kinds of movies they tend to make are easy sells to audiences in Russia, China and other territories, where the action transcends any language.

Filmmakers, however, are sometimes taking a risk with older actors. They might get injured more easily, be at increased danger of illness or be unable to perform certain physical tasks a younger actor might.

Productions may also have to pay more for insurance.

“It’s an issue,” Ray Ossenkop, managing director of entertainment at insurance brokerage Truman Van Dyke, tells The Post. “It may not be anything, depending on the health of an actor, or it could be more than twice as much. There’s no fixed cost.”

So how much longer can these stars hang around? Stallone, for one, has talked about next remaking two of his ’80s shoot-em-ups, “Cobra” and “Tango & Cash.”

Peterson says Sly appears to be nowhere near retirement. “He’s not going from shooting to shuffleboard,” the trainer says.

“Acting is selling,” says producer Burn, also an actress who appeared with Stallone in 2014’s action all-star vehicle “The Expendables 3.” “As long as you can sell the audience that you are that character, then we’ll keep buying it.”