Thursday marked 50 years since Mick Jagger played his first gig with a band called the Rolling Stones, and the group is marking its half-century with no letup in its productivity or style. Now in their late 60s and early 70s, the band members celebrated the anniversary by attending a retrospective photo exhibition at London’s Somerset House — and looking to the future by rehearsing for new gigs. Jagger, Keith Richards, Ronnie Wood and Charlie Watts mingled with celebrities from Mick Hucknall to Tom Stoppard at a launch party for the exhibition, which charts the band’s career from their first official photo shoot to their monster stadium tours. The band got together 50 years to the day after the young R&B band played London’s Marquee Club. Taking a name from a song by bluesman Muddy Waters, they were billed as “The Rollin’ Stones.” The lineup for the gig was vocalist Jagger, guitarists Richards and Brian Jones, bassist Dick Taylor, pianist Ian Stewart and Mick Avory on drums. Taylor, Stewart and Avory soon left the lineup; drummer Watts joined in 1963 and guitarist Wood in 1975. The band had its first hit, a cover of Chuck Berry’s “Come On,” in 1963, and soon became one of the world’s biggest and most influential rock acts, rivaled only by The Beatles. The Beatles split up in 1970, but the Stones are still going strong — something Jagger, 68, says he could never have imagined at the time. Richards told the BBC that his biggest regret in the last 50 years was the drowning death in 1969 of Brian Jones, but that on the whole the band’s career has been “an incredible adventure.” The Stones have sold more than 200 million records, and Richards says more live shows are on the way.