Queen of the United Kingdom
Years served: 70
Queen Elizabeth, mother of Prince Charles and grandmother to Princes William and Harry, was not only the oldest world leader on this list, she was also the second-longest reigning monarch in world history and the longest-reigning in the history of her country, despite recent health scares. Upon her 70-year anniversary, the queen surpassed Thailand's King Bhumibol Adulyadej, who reigned from 1946 until his death in 2016. She was behind only France's King Louis XIV, who gained his title at just 4 years old in 1643. Louis XIV was king until his death in 1715 — reigning for a total of 72 years and 110 days. Elizabeth became queen in 1952 and was crowned in 1953 after her fate was dramatically changed by a love affair between her uncle, King Edward VIII, and American divorcee Wallis Simpson. Edward abdicated the crown to follow his heart and be with Simpson, and Elizabeth suddenly found herself heir to the throne behind her father, George. When King George VI died in 1952, she officially became Queen Elizabeth II.
Related: 17 Reasons You Don't Really Want to Be a Royal