What's the matter with kids today? Nothing. At least not with the ones loitering outside the Lincoln Theater during a New World Symphony intermission. The NWS's artistic director, Michael Tilson Thomas, calls classical music "a rare and wonderful thing." The same can be said of the NWS, the baby of all symphonic orchestras. The NWS, which recently celebrated its fifteenth-anniversary concert, is also known as America's Orchestral Academy. Its 85 members are recent graduates of the nation's finest conservatories and university music schools, and they can stay only three years before they must find jobs with orchestras across the land. MTT describes NWS as "an investment in a whole new generation of musicians." But that doesn't mean the NWS sounds like a bunch of juveniles. On the contrary, a young tuxedo-clad horn player walks into a gelato shop after a recent program of Mozart's Six German Dances, Hindemith's Concert Music for Strings and Bass, and Mahler's Symphony No. 4. Seated at a table was a classical connoisseur who had witnessed the performance and couldn't constrain himself when he noticed the lad's duds and horn case. "That's the best orchestra I've heard in a long, long time," he exclaimed. "You should be proud of yourself." The young musician smiled and replied right on cue: "Thanks for coming." Cultivating such audience appreciation is one key to NWS's success. Another key is MTT's insistence his young musicians learn state-of-the-art techniques for mastering auditions, the classical musician's number-one source of fear and loathing. During their tenure they receive a modest stipend, but more important is the payment in kind that comes from working with a conductor of MTT's stature.