Complex sentence: By the age of 16, Rajković was a regular in the Serbia under-21 national team, later becoming the youngest player ever to appear in a qualifying match of the UEFA European Under-21 Football Championship.
Split into simpler sentences: By the age of sixteen he was a regular in the Serbia national under-21 football team. He was the youngest player ever to appear in a qualifying match of the UEFA European Under-21 Football Championship.