With two septuagenarians -- 77-year-old Joe Biden and Donald Trump, 74 -- facing off for the US presidency, a look at the world's oldest leaders. Whoever wins will be the oldest G20 leader after Saudi Arabia's 84-year-old King Salman. The world's oldest elected leader is Cameroon's 87-year-old president Paul Biya. Although "The Sphinx" has been in power for nearly four decades, he is a mere stripling compared to Malaysia's 95-year-old Mahathir Mohamad, who only stepped down as prime minister in February. With 68 years on the throne, Britain's Queen Elizabeth II holds the record for the longest period in office. Now 94, she is the head of state of 16 countries including Canada and Australia. Cuba's Raul Castro -- the younger brother of the late leader of the country's revolution, Fidel Castro -- still holds the reins of power at 89 even though he relinquished the presidency last year. Kazakhstan's 80-year-old Nursultan Nazarbayev, its former communist chief, is also the Central Asian country's de facto leader even though he retired as president last year. Laos' communist President Bounnhang Vorachith is 83, although he is a relative new boy having only risen to power in 2016. Africa may be the continent with the youngest population but its leaders don't reflect that. Cameroon has its elderly "Sphinx" but in Zimbabwe it is "The Crocodile" Emmerson Mnangagwa, 78, who rules the roost, so called because of his political savvy and bite. Guinea's Alpha Conde is 82, having won a highly disputed third term last month, while Congo's Denis Sassou Nguesso at nearly 77 has spent a total 37 years in power. Also 77 is Nigeria's President Muhammadu Buhari, who returned to power in the 2015 election after previously leading a military coup in 1983. Saudi Arabia's King Salman, 84, does not have such concerns but has delegated much of the responsibility for his oil-rich kingdom to his son, Prince Mohammed bin Salman. The Middle East has a cluster of some of the world's most powerful octogenarians. Iran's supreme leader Ayatollah Ali Khamenei is 81 while Lebanon's President Michel Aoun is 85 and Mahmud Abbas, the head of the Palestinian National Authority, is nearly 85. bur-fg/erc