ROME — A film about a bevy of lively ladies over 80 being catered to by a boyish middle-aged boozer is doing gangbuster biz at the Italo box office and becoming an international cause celebre.

Roman multihyphenate Gianni Di Gregorio, 59, was fretting over whether “Mid-August Lunch,” his first feature, would make the cut for the Venice Critics Week.

Then last month Di Gregorio took the Lion of the Future for best first work at the Venice fest, marking the first time the nod went to someone his age. A simple tale, gentle in tone but biting enough for belly laughs, “Lunch,” which cost $600,000 to make, and stars mostly nonpros, has since gone on to gross more than $2 million locally so far via Domenico Procacci’s Fandango.

Di Gregorio stars as Gianni the son of Valeria, played by formidable 93-year-old Valeria De Franciscis.

“Lunch” also sparked bidding wars on the Lido and has been sold by Fandango Portobello to a slew of territories, including France (Le Pacte) and Germany (Pandora), with a U.S. sale now deemed a distinct possibility.

Di Gregorio, a thesp, scribe and longtime Matteo Garrone collaborator — most recently as a writer on “Gomorrah” — based “Lunch” on his own experiences living with his mother, “a woman with an overwhelming personality,” he says.

At the time, he had been tempted to accept lucrative offers from neighbors asking him to take care of their moms as well, which inspired him to tell the tale of a sixtysomething man living with his mamma strong-armed into taking in some elderly houseguests over Italy’s midsummer holiday weekend in order to pay the rent.

Garrone shepherded the project after Di Gregorio got cold-shouldered by other producers.