1. King William IV

Age at accession: 64 years, 10 months, 3 days.

Reign: 1830–1837.

His father was King George III, whose next-youngest son was King George IV. William joined the Royal Navy when he was just 13, and he saw action against both Spanish forces and American forces (in New York, during the Revolutionary War). Because of his naval background, he was called the “Sailor King.” Parliament passed both the Reform Act and the Abolition of Slavery Act during William’s reign. William IV was succeeded by Queen Victoria.