Ninety-four-year-old former President Jimmy Carter says he does not believe he could have managed the most powerful office in the world at 80 years old.
Mr Carter did not tie his answer on Tuesday to any of his fellow Democrats running for president.
But two leading 2020 candidates, Joe Biden and Bernie Sanders, would turn 80 during their terms if elected.
Mr Biden is 76. Mr Sanders is 78.
Mr Carter’s remarks came during his annual question-and-answer sessions at the Carter Centre in Atlanta.
The 39th US president was 56 when he left office in 1981.
Mr Carter says he voted for Mr Sanders in the 2016 primary.
He says he is undecided now but will vote for the Democratic nominee against President Donald Trump.