It’s far too early, of course, to say what James Levine’s ultimate legacy as music director of the Boston Symphony will be. But already, it is clear that Levine is having a rejuvenating effect on the BSO’s famed Tanglewood Music Center, which brings many of the country’s top young musicians together for several weeks of summer instruction and orchestral training. “For years, the presence of Tanglewood icon Leonard Bernstein made the TMC the place to be, despite the competition. And that phenomenon is happening again because Levine is in the Berkshires.”