Complex sentence: He started singing when he was just three years-old and joined a children's choir at age four; when he was 9, he was recognized as the best child voice in Europe at a contest in Salzburg, Austria.
Split into simpler sentences: He started singing when he was just three years-old and joined a children's choir at age four. When he was 9, he was recognized as the best child voice in Europe at a contest in Salzburg, Austria.