Complex sentence: Mulders started at A.V.V. Zeeburgia at the age of four until he was scouted by Ajax at the age of eight and trained at the Ajax Academy until he was 16.
Split into simpler sentences: Mulders started at AVV Zeeburgia at the young age of four until he was scouted by Ajax at the age of eight. He trained at the Ajax Youth Academy until he was 16.