Complex sentence: Peter Anthony Allen was 21 years old when on 13 August 1964 he became one of the two last people in the United Kingdom to be executed, when he was hanged for the murder the previous 7 April of John Alan West.
Split into simpler sentences: Peter Anthony Allen was twenty-one years old when on 13 August 1964 he became one of the two last people in the United Kingdom to be executed. He was hanged for the murder the previous 7 April of John Alan West.