Close your eyes during a performance by the New York Youth Symphony, and you might have trouble believing that you’re listening to a group of fresh-faced youngsters—except, perhaps, for the fact that each concert features a newly composed piece, something practically no other ensemble in town can boast. This afternoon the orchestra surrounds a string quartet made up of players from the American Contemporary Music Ensemble (ACME) in the premiere of Timothy Andres’s Senior. Ryan McAdams also conducts Chávez’s Sinfonía India and Brahms’s Symphony No. 1.