At the Regal Cinemas LA Live Stadium 14 in downtown Los Angeles , a mixed-age audience that filled more than two-thirds of the seats clapped and cheered loudly as Gustavo Dudamel , the orchestra 's charismatic 29-year-old conductor , led the orchestra through Adams ' " Slonimsky 's Earbox , " Leonard Bernstein 's Symphony No. 1 ( " Jeremiah " ) and Beethoven 's Symphony No. 7 .