A few days ago Queen Elizabeth passed Johann II of Liechtenstein to be the third longest reigning monarch (see here).
A summary of the top 4:
4) Johann II, Liechtenstein ruled from Nov 12 1858 until his death on Feb 11, 1929. When he became King he was 18. He was king for 70 years, 91 days.
3) Queen Elizabeth II (thats a two, not an eleven) ruled from Feb 2, 1952 until now. When she became Queen she was 25. I am writing this on May 10 at which time she ruled 70 years, 94 days.
2) Bhumibol Adulyadej (Thailand) ruled from June 9, 1946 until Oct 13, 2016. When he became King he was 19. He was king for 70 years, 126 days.
1) Louis XIV ruled from May 14, 1643 until Sept 1, 1715. When he became King he was 4 years and 8 months. He was king for 72 years, 110 days.
Johann, Elizabeth, and Bhumibol started their reigns a bit young (they would have to to have ruled so long) but their first day of their reign they knew what the job was, what they are supposed to do etc.
Here is my complaint: Louis XIV being king at the age of 4 years 8 months should not count (someone who proofread this post wondered if 4 years 9 months would count. No.) Shouldn't we define the reign of a king as the point at which he can make real decisions as king? Or something like that.
For the record of the longest marriage there is a similar problem. The three longest marriages are legit in that the people got married at a reasonable age (I think all were married after they were 17). The fourth longest marriage of all time involved two people that were married when they were 5. That should not count (see here).
Is there a way to define monarch's reigns and also marriage length so that it corresponds to our intuitions?
In Math we can use rigorous definitions but in English its harder.