Complex sentence: He started at the age of 8, however never pursued a career, a brief return at the age of 16 proved unfruitful and he never took to karting, instead focusing on a career in cars.
Split into simpler sentences: He started at the age of 8, however never pursued a career. A brief return at the age of 16 proved unfruitful and he never took to karting, instead focusing on a career in cars.