Answer
Semmy Schilt was 23 years old when he started his professional career. He made his debut in 1999 and went on to become one of the most successful heavyweight kickboxers of all time. Schilt won the K-1 World Grand Prix in 2005 and 2008, and he was the K-1 Heavyweight Champion from 2005 to 2007. He retired from professional kickboxing in 2013.