Complex sentence: Known as "Cowboy", McCormack started boxing in 1950 at the age of fifteen, and by 1956 he held the Scottish and A.B.A. Light Middleweight titles in 1956 as an amateur.
Split into simpler sentences: Known as "Cowboy", McCormack started boxing in 1950 at the age of fifteen. By 1956 he held the Scottish and A.B.A. Light Middleweight titles as an amateur.