972. 25 years, 123 days

From AP:

Since leaving the White House in 1981, Jimmy Carter and Walter Mondale have set the standard for public service after holding the nation's highest offices. Today, they set the record for longevity as well: the Democrats have lived 25 years, 123 days since leaving office.

Carter was 52 and Mondale was 48 when they took office in 1976. Carter is 81 and Mondale is 78 now.

Trivia question: Whose after office longevity record did they beat? Answer at the full AP story here.