Complex sentence: On leaving school, aged 15, he became a ladies hairdresser eventually running his own salon until the age of 21 when he decided to try to go back to college, eventually qualifying as a teacher.
Split into simpler sentences: On leaving school, aged 15, he became a ladies hairdresser eventually running his own salon until the age of twenty-one. He decided to try to reclaim the education he had missed at school and went back to college, eventually qualifying as a teacher.