Part of conductor's job is to be a glorified traffic cop, to make sure the music doesn't come apart at the seams. But even more important than that, it's to be a leader and a unifier, to convey an artistic vision to the orchestra. If the first part of the job description seems better-suited to younger people, it's the second part that has historically swung the baton in favor of the senior generation. Pablo Heras-Casado is a 34-year-old Spaniard who seems to have both halves in good order. Today, he leads the Los Angeles Philharmonic in Stravinsky's Firebird Suite.