Complex sentence: In 1919, at the age of 13, he was allowed to enter the Petrograd Conservatory, then headed by Alexander Glazunov, who monitored Shostakovich's progress closely and promoted him.
Split into simpler sentences: In 1919, at the age of 13, he was allowed to enter the Petrograd Conservatory, then headed by Alexander Glazunov. Glazunov monitored Shostakovich's progress closely and promoted him.