MANSFIELD, MASS. — Three young teenage girls sidled down the row, prodded toward an empty bank of seats by their impatient parents. They were late. Elton John was already halfway through his second song on a muggy August night at the Great Woods Center for the Performing Arts. As John pounded the keys of his luxuriously long concert grand piano, singing ''I Guess That's Why They Call It the Blues,'' the girls slipped into their seats, their arms folded and their faddishly barretted heads sunk low. ''He's just not cool,'' their expressions seemed to say. This, after all, is the guy who sings those kiddie ''Lion King'' songs. But over the next three hours, John's powerful piano and spontaneously playful stage antics brought these girls' heads up as they noticed that this guy wasn't just another of their parents' easy-listening favorites. Throughout, John demonstrated why, from the '70s onward, he keeps winning over the generations. Indeed, the girls had missed John's hard-driving and appropriate opening song: ''I'm Still Standing (After All These Years).'' Twenty-five years after ''Your Song,'' his first US top-10 single, John's boyish energy still flows. Following many numbers, a smiling John would leap on top of the piano bench, shimmy across the piano top, land on the stage, introduce the next song, and slide back to the keyboard. But the skeptical teens did arrive at one of the show's low points. The first several songs had a tinge of nervous energy perhaps more typical of John's '70s concerts, in which he cavorted on stage in court-jester-like costumes with his signature bright-colored thick glasses. The nervousness was compounded by a trio of male guitarists straight out of the '80s: Their long, blond Van Halen-esque hair blew in the wind of fans positioned in front of each of them. And their long guitar and bass strums almost conflicted with John's masterly piano. It wasn't until later, when he was alone on stage, that John exhibited the mature style he has evolved over the decades. During a long, piano-only intro, the heavily miked keyboard resonated powerful bass chords and tinkled with John's spritely keyboard antics. Once, he stretched out both legs and propped his feet just beyond the highest notes - fingers flying all the while - and playfully smiled at the audience. Even the girls got in a few claps. Another highlight: ''Rocket Man.'' Led by longtime percussionist Ray Cooper, the song brought together all the elements of John's show. Cooper, who stood on an elevated platform at the back of the stage surrounded by bongos, cymbals, and a 6-foot-tall gong, is more of a showman even than John. He had the audience chanting ''Coop, Coop,'' after a long, sometimes-frenzied solo. Spurred by Cooper's antics, John went wild: He kicked out the piano stool, knelt down, and - playing all the while - maneuvered himself onto his back underneath the piano with one arm reaching up, still playing the keys. The teenagers were impressed. John had been wearing a simple cream suit. But for the three encores, he strode back on stage with a sequined jacket and a wilder look in his eye. During ''Saturday Night's All Right for Fighting,'' as the guitars wailed and John pounded, the teens' sideways glances bespoke the response: ''Mom, Dad, you listen to this guy?'' ''Cool.'' But some older folks in the crowd, who before had been bouncing and twisting, seemed a bit overwhelmed. Some stood in slightly awkward silence as John's '70s psychedelic colors and sounds screamed forth. In the last encore, however, it was the simple and mature John that prevailed. In solid black jumpsuit, he waxed a bit sappy about his appreciation for the fans' longtime support. Then, as he always does these days, he dedicated the final tune, ''The Last Song,'' to those medically diagnosed with AIDS. As the girls slid toward the aisle, their faces betrayed a mix of bewilderment and amazement. Maybe now when Elton John comes on in the car with Mom or Dad, they'll listen a little closer.