Complex sentence: He was born in Pest, Austria-Hungary, where he began the study of music at a very early age, when only seven years old he could pick out at the piano melodies which he had heard in the theatre.
Split into simpler sentences: He was born in Pest, Austria-Hungary, where he began the study of music at a very early age. When only seven years old, he could pick out at the piano melodies which he had heard in the theatre.