It’s after midnight on Main Street and the Set Back Inn is jammed, and in the back of the room, a 51-year-old medical social worker and mother of three is living her other life, and loving it.

She is singing. She is swaying. She is belting out the soulful strains of Joe Cocker’s “The Letter,” skillfully backed by four musical comrades, the latest gig of a local band that has become a tight-riffing, sweet-sounding River Towns phenomenon.

The band is called Juke Joint, and to point out that its five members all qualify for AARP membership, with a collective age of more than 250, is completely beside the point.

Because they can really play.

“You need to find joy, and live your life with passion,” says Mary Kohrherr, the social worker/lead singer, who lives in a 19th century Victorian on Benedict Avenue. “We’re not doing this to become rock stars. We’re doing it to have fun.”

Some three decades ago, Kohrherr was the singer for a rock band in Norfolk, Virginia. She moved to New York, tended bar while she waited for her big break, and wound up falling in love and marrying David Titcomb, a bass trombonist, who is also the orchestra manager for the New York City Opera. They moved to Tarrytown almost 20 years, and three children, ago.

“I always said when I turned 50 I was going to sing in another band, and now I am,” Kohrherr said, smiling.

Juke Joint consists of Ray Ovetsky (guitar), Gene Eames (bass), Ed Bettinelli (drums) and Jeff Schoen (keyboards), who is Kohrherr’s Tarrytown neighbor. Ovetsky lives in Irvington, Bettinelli in Dobbs Ferry. Eames resides in Newtown, Conn. That they all managed to find each other has to do with a walk in the woods, craigslist and serendipity.

“It’s just a great mix of musicians, and we happen to really like each other,” Schoen said.

About 14 months ago, Kohrherr was out for an autumn walk in Rockwood Hall with her friend, Anita McGovern. Kohrherr mentioned her interest in finding a midlife spirit booster – and getting back into singing.

“You just need to do it,” McGovern told her.

Kohrherr needed no further encouragement. She turned to craigslist, and put an ad out for bands interested in a lead singer:

Energetic, mature. experienced female vocalist looking to play out once a month in the Westchester area. Looking for a fun, creative band to gig with.

The listing led to a couple of auditions that didn’t go anywhere, but ultimately connected Kohrherr with Ovetsky, who is old friends with Bettinelli, who had played with Eames. A Tarrytown friend told Kohrherr about Schoen, who spent three years in the 70s playing keyboards for Wilson Pickett.

“It all came together like it was meant to be,” Kohrherr said.

Juke Joint practices about four hours a week, in Bettinelli’s drum studio on the river in Dobbs Ferry. It had its first gig at J.P. Doyle’s on Beekman Avenue in Sleepy Hollow last May. Brian Doyle, the owner, is an active supporter of various community initiatives. Kohrherr showed up at the bar with a CD, a photo and the relentlessness of an aspiring artist looking for gig. She got politely rebuffed, and showed up again…And again.

Kohrherr has worked in the EPTA for the Tarrytown schools, coached her kids in basketball, soccer and softball and for the last two years, been a regular at a pre-dawn fitness “boot camp” in Pierson Park. She is one of those people who knows everybody, and does everything. She could probably talk a cat out of chasing a mouse.

“After she came by three times, I finally said, ‘I’ve got to get this lady in here to get her off my back,’” Doyle said. He laughed. “It was one of the best decisions I’ve ever made. They pack the place every time they play. I thought it was a hokey little band. I didn’t realize Mary had so much talent.”

Juke Joint has since played the Bridgeview Tavern, Pete’s Saloon in Elmsford, and the Set Back Inn, where they had what may have been their most rousing performance yet last month, when they were joined by Titcomb and four other horn players from the New York City Opera and New York City Ballet. Consisting of two trumpets, two trombones and a cimbasso (an Italian instrument that is a cross between a bass trombone and a tuba), the horn section hustled straight from Lincoln Center and “The Nutcracker” to get to Main Street in time to play an entire set that included “Son of a Preacher Man” by Dusty Springfield, a couple of songs by Sharon Jones and the Dap-Kings and “The Letter.”

Without so much as five minutes of rehearsal time, the guest horn players added a polished, brassy backbeat, and played to rave reviews before a crowd that included people who came by after the Southside Johnny concert, just up the street at the Music Hall.

“They were incredible musicians to play with,” Schoen, the keyboard player, said.

Juke Joint’s next gig will be back at J.P. Doyle’s, on January 27. Kohrherr, social worker by day and rock mama by night, won’t have to do any sales work this time. The local phenomenon known as Juke Joint has a loyal, and growing, fan base.

“I have such a great life, and the music just makes it that much richer,” Kohrherr said. “We just try to enjoy making music, and try to make it enjoyable for everybody else, too.”