Andris Nelsons will be the new music director of the Boston Symphony Orchestra, the orchestra announced Thursday. The appointment of Nelsons, at 34, follows the trend of many U.S. orchestras in hiring relative youngsters. He will be music director-designate in the coming (2013-14) season, and succeeds James Levine, who was music director from 2004 to 2011. Latvian-born, Nelsons has been music director of the City of Birmingham Symphony Orchestra since 2008. His rapport with the BSO is not long-established; he first led the group in 2011.