Charles V abdicated his throne at the age of 56 between 1555-1556 because after almost forty years of ruling an empire (which included the Holy Roman Empire and the Spanish Empire) a series of severe and complicated health issues made him decide to retire.
Apart from ruling one of the most powerful reins by that time, Charles decided to finally abdicate his throne after he had many militaristic, political and especially health problems in his life. For example, he suffered from chronic indigestion, epilepsy as well as gout that was caused by his Hapsburg jaw condition. This disease deformed his jaw completely.