Washingtonpost--  At 92, Mahathir is almost three decades older than Najib. He was Malaysia's longest-serving prime minister, having held power from 1981 to 2003. If he wins the next election, scheduled to be held before August, Mahathir will become the world's oldest head of state or government.

Include hereditary rulers and autocratic leaders and that's quite a competitive title. Britain's Queen Elizabeth II, 91, holds the distinction of being the oldest world leader — a title she took from Zimbabwe's Robert Mugabe when he was ousted from office last year at age 93. Beji Caid Essebsi, Tunisia's 91-year-old president, is the oldest-serving elected world leader.

Mahathir's decision to run is a reflection of the state of Malaysian politics and, in particular, his public opposition to former ally Najib, who is linked to a multibillion-dollar financial scandal involving a state development fund. If elected, Mahathir is expected to use his power to pardon another major opposition figure, Anwar Ibrahim, who is serving five years on sodomy charges that his supporters say are politically motivated.

If he is pardoned, 70-year-old Ibrahim could run for prime minister — potentially succeeding Mahathir, a former-ally-turned-rival-turned-ally-again.

At the same time, the Malaysian election fits into a global conversation about how old world leaders should be. That debate has been especially pronounced in the United States, where then-70-year-old Donald Trump faced off against 69-year-old Hillary Clinton in the 2016 election — the two oldest candidates in a U.S. presidential election, with the exception of incumbent Ronald Reagan in 1985.