Complex sentence: On the succession of Gordon Brown as Prime Minister (2007), Miliband was promoted to Foreign Secretary, at 41, the youngest person to hold the post since David Owen 30 years earlier.
Split into simpler sentences: On the succession of Gordon Brown as Prime Minister in 2007, Miliband was promoted to become Foreign Secretary. At 41, he became the youngest person to hold that office since David Owen 30 years earlier.