Complex sentence: Mills, who played as a defender and as a midfielder, started his career with First Division club Nottingham Forest and he became the club's youngest league player after making his first team debut at the age of 16 in 1978.
Split into simpler sentences: Mills, who played as a defender and as a midfielder, started his career with First Division club Nottingham Forest. He became the club's youngest league player after making his first team debut at the age of 16 in 1978.