Complex sentence: He began riding motocross at the age of 8 years old, after his dad bought him a dirtbike, Nate entered and won several motocross races and won them as a kid before entering the world of Freestyle Motocross when he was 17.
Split into simpler sentences: He began riding motocross at the age of 8 years old after his dad bought him a dirtbike. Adams entered and won several motocross races and won them as a kid before entering the world of freestyle motocross when he was 17.