Lest it go unsaid, Kronos Quartet are a case study in aging gracefully. With their days of space suits and Hendrix covers behind them, Kronos finesse the line between old and new with a style rarely evinced by your average rock band, let alone a 35-year-old string ensemble. Tonight, they perform world premieres that hammer this point home. John Adams, that longstanding titan of American minimalism, brings gravitas to the program with his compact, exhilarating new work, Fellow Traveler. Meanwhile, twentysomething Aviya Kopelman — whose digitally tweaked piece is the fruit of the quartet's Under 30 commissioning project — shows us what "new music" means to next-gen composers who've lived fewer years than Kronos have played.