Mozart was 35 when he died in 1791. He had been writing an exceptionally long piece of music, and was drunk at the time so that wasn't a very wise decision on his part.
The assumed reason for his death was that Mozart believed his father had come back from the dead, and that he was telling Mozart to write a piece of music for him which Mozart called The Requiem. Since the requested piece was so large, It eventually drove Mozart crazy, he drank a lot-- presumably to forget about his troubles-- and he soon died. Mozart left The Requiem unfinished.