Jimmy Carter and his wife, Rosalynn, were married for 77 years. Their relationship may have helped them live so long.

Jimmy and Rosalynn Carter.

Bettmann/Getty Images; Michael Tran/Getty Images

• Jimmy Carter has died at 100: the average life expectancy for men in the US is 73.5.
• Carter's 77-year marriage could have been key to his long life, longevity expert Dan Buettner said.
• Research suggests married people, especially men, live longer than their unmarried counterparts.

Former President Jimmy Carter, who died at age 100 on Sunday, said marrying his wife Rosalynn at age 21 was the "best thing" he ever did.