If 24 seems a tad young to be getting married, blame it on the fact that singer-songwriter Jessie Baylin sowed her oats long ago. "At 16, I was dancing on the bar at [Chelsea club] Suite 16, so I definitely grew up faster than I would have had I stayed in New Jersey and gone to, like, public school," says Jessie, who's engaged to Nathan Followill, drummer for band Kings of Leon. Instead, the chanteuse—whose smoky voice on her new album Firesight is reminiscent of Stevie Nicks and Norah Jones—attended the Professional Children's School on the Upper West Side and remains close with classmate Scarlett Johansson. "I watch Gossip Girl and there are definitely moments when I've thought, 'Oh my God, that's real,' " laughs Jessie. Back then, "I was like, 'Let's have Cosmos after school!' " It was over different cocktails in L.A. (where Jessie had moved after graduation so she could try her luck singing on the club circuit) that another star, John Mayer, came into her life. "We were hanging out with some mutual friends at the Four Seasons and I just started singing," she recalls. "He really loved my voice and has championed me ever since. He's taken me along for a bit of his ride." That journey includes a performance slot on John's upcoming "Mayercraft" cruise to Cabo San Lucas—a four-day floating music festival that sets sail in March. "John told me it's a giant booze cruise," says Jessie. "It's gonna be amazing." OK, so maybe she's got some oats to sow yet.