21 August 1982
In 1982, the small kingdom of Swaziland was shocked when His Majesty, King Sobhuza II, passed away at the age 83 at the Embo State House. The king died after a successful reign of sixty years and was the longest reigning monarch in the world at the time. King Mswati III ascended the throne in 1986 at the age of 18 - as the youngest reigning monarch in the world.
References

Swaziland National Trust Commision,History of his Majesty,King Sobhuza 11,[online],Avaliable at www.sntc.org.sz [Accessed: 21 August 2013]|

The Star, Thursday August 21, 2003. Milestones