Queen Elizabeth II, United Kingdom
Born: 21 April, 1926 (aged 92).
Queen Elizabeth II visits Marlborough House. Source: AAP
Queen Elizabeth II is the world's oldest head of state and UK's longest reigning monarch.
After spending time in the Auxiliary Territorial Service in World War II, she went on to assume the throne in 1952.
Material from Buckingham Palace says, "known for her sense of duty and her devotion to a life of service, she has been an important figurehead for the UK and the Commonwealth during times of enormous social change".
She is also reportedly history's most widely travelled head of state - visiting more than 110 countries.
President Beji Caid Essebsi, Tunisia
Born: 29 November, 1926 (aged 91).
Tunisian president Beji Caid Essebsi during a press conference in Italy. Source: AAP
Beji Caid Essebsi is Tunisia's first freely and directly elected leader.
Mr Essebsi came to power in 2014 at age 88, winning the first presidential election after the Tunisian Revolution which ousted autocrat Zine El Abidine Ben Ali.
He previously served as the country's minister of foreign affairs from 1981 to 1986, and prime minister from February 2011 to December 2011.
"I am committed to be president of all Tunisian men and women without exclusion," the secular Mr Essebsi said at his swearing-in ceremony. "There is no future without consensus among all parties and social movements."
Some cite Tunisia's fragile democracy as one of the more positive outcomes of the Arab Spring.
Sheikh Sabah al-Ahmad al-Jaber al-Sabah, Kuwait
Born: 16 June, 1929 (aged 88).
The Emir of Kuwait Sheikh Sabah Al Ahmad Al Sabah visiting the White House in 2017. Source: AAP
Sheikh Sabah was Kuwait's foreign minister for 40 years, including during the Iraqi invasion in 1990.
He went on to be prime minister then emir. During his time in power, Mr Sabh has continued the country's pro-West stance and pursued limited reform.
On a recent trip to the US, President Donald Trump reportedly complained about how Mr Sabh's official plane was bigger than Air Force One.
President Paul Biya, Cameroon
Born: 13 February, 1933 (aged 85).
Cameroonian President Paul Biya. Source: AAP
Paul Biya is one of Africa's longest-serving leaders, having been in power for 35 years.
While Mr Biya has had some economic success, his time in power has also been marked by corruption and a reversal of democratic gains.
Mr Biya's taste for alpaca suits and silk ties - and repeated and often lengthy absences, especially to Switzerland - have been a source for criticism in a nation where more than a third of the population still survives on less than $3 a day.
Emperor Akihito, Japan
Born: 23 December, 1933 (aged 84).
Emperor Akihito delivers a speech during the opening ceremony of a Diet session in Tokyo. Source: AAP
Akihito came to power in 1989. He has spent much of his reign trying to address the legacy of World War II, which was fought in the name of his father Hirohito, and consoling victims of disasters or other woes.
Akihito will step down on April 30, 2019 - the first abdication by a Japanese monarch in about two centuries.
The news has been met with shock around Japan.
"(He) thought tirelessly about the people," 72-year-old Japan resident Taeko Ito recently told Reuters.
"Now he is elderly and I wish from my heart that he can have a rest."
- Additional reporting: Reuters, AFP
