Complex sentence: Retuinskih began his training at 7 years old learning gymnastics, at the age of 12, he began studying boxing, at 14, Sambo and Combat Sambo and at age 19, Judo and Boxing.
Split into simpler sentences: Retuinskih began his training at 7 years old learning gymnastics. At the age of 12, he began studying boxing, at 14, Sambo and Combat Sambo and at age 19, Judo and Boxing.