Age is not just a number when you’re the leader of the free world, according to former President Jimmy Carter.

Carter, who turns 95 on Oct. 1, said Tuesday that managing the job of U.S. president even at age 80 would have been too demanding for him. (See the clip below.)

“If I were just 80 years old, if I was 15 years younger, I don’t believe I could undertake the duties I experienced when I was president,” Carter, the longest-lived U.S. leader, said during his annual report at the Carter Center in Atlanta. “You had to be very flexible with your mind. You have to be able to go from one subject to another and concentrate on each one adequately and then put them together in a comprehensive way.”

Carter’s remarks began on a light note when he was asked if he’d consider running for a second term. “I hope there’s an age limit,” he replied, laughing.

Carter did not mention that two of the leading Democratic 2020 candidates, former Vice President Joe Biden, 76, and Sen. Bernie Sanders (I-Vt.), 78, would turn 80 in office if elected.

President Donald Trump, 73, became the oldest newly elected president at 70 on his inauguration day.