Complex sentence: In 1993, when he was 10 years old, he went to Bayer 04 Leverkusen, which he left 5 years later to play for Fortuna Düsseldorf where he became a professional football player in 2001.
Split into simpler sentences: In 1993, when he was ten years old, he went to Bayer 04 Leverkusen. Which he left Bayer 04 Leverkusen five years later to play for Fortuna Düsseldorf where he became a professional football player in 2001.