Noting the fuss over Elliott Carter’s impending centenary, partisans of Milton Babbitt’s music are getting their ducks in a row. Mr. Babbitt is 92 and apparently in good health, and 2016 will be upon us in no time. So on Wednesday evening the Miller Theater devoted a Composer Portraits program to Mr. Babbitt’s string quartets. The composer was on hand for an informal postintermission discussion with James Levine, long an avowed Babbitt fan. And Mr. Carter was seen in the audience, probably to hear what the young whippersnapper was up to.