The Nashville Symphony Orchestra's choice for a new, permanent music director is still anybody's guess, but the national trend is clear: important orchestras around the country are taking chances, selecting young and vigorous new leaders. Just yesterday, the New York Philharmonic picked Alan Gilbert, 40, to be its new music director. That decision follows the Los Angeles Philharmonic's recent naming of 26-year-old Venezuelan Gustavo Dudamel as its conductor. Will Nashville, which tapped 62-year-old veteran Leonard Slatkin as its caretaker conductor, take a chance with youth? We may find out soon enough.