ON ANY night on stages across America, fans line up to see Tom Petty, Prince, Frank Sinatra and other beloved singers perform. Tribute acts bring them to life again. Audiences are typically 45 and older, and eager to turn back time, says Michael Twombly, 59, cofounder of Music Zirconia, a booking agency. Acts aren’t limited to deceased performers or broken-up bands, either. “The industry has never been bigger,” Twombly says. —Jeffrey Lee Puckett