Complex sentence: As a teenager, Vandenberg competed in many martial arts competitions winning the majority of them and left school at 16 to enroll in a Kunto training course in Okinawa, Japan.
Split into simpler sentences: As a teenager, Vandenberg competed in many martial arts competitions. He won the majority of them and left school at age 16 to enroll in a Kunto training course in Okinawa, Japan.