Lambert was a prodigy : he was composing music for orchestra when he was only 13 , and when he was 20 he was asked to write a ballet for Serge Diaghilev 's world-famous Ballets Russes ( Romeo and Juliet ) . Lambert was a prodigy , writing orchestral works from the age of 13 , and at 20 received a commission to write a ballet for Sergei Diaghilev 's Ballets Russes ( Romeo and Juliet ) .