Complex sentence: He played soccer as a goalkeeper when he was young and started train in martial arts when he was in the ninth grade in school and transitioned to MMA later where he turned pro in MMA when he was 18.
Split into simpler sentences: He played soccer as a goalkeeper when he was young and started train in martial arts when he was in the ninth grade in school. He transitioned to MMA when he was a tennager and turned pro in MMA when he was 18.