Mozart's greatest works were written in the last five years of his life

Wolfgang Amadeus Mozart died at 35 in 1791, and while he began composing at age 5, his most celebrated works — Don Giovanni, The Magic Flute, Symphony No. 40, the Jupiter Symphony, and his Requiem — were all completed between 1787 and 1791. His output was extraordinary in quantity (over 626 compositions) and quality, but the myth of effortless genius is misleading: Mozart's notebooks show extensive revision and correction. He was also deeply in debt at the time of his death, relying on loans from friends, and was buried in an unmarked common grave as was standard for his class in Vienna.

Source: Solomon, Mozart: A Life (1995)