[Pete] Buttigieg, 37, Reps. Tulsi Gabbard and Eric Swalwell, 38, and Rep. Seth Moulton, 40, are all attempting to overtake Roosevelt's record as America's youngest president.
[Joseph] Biden, Sen. Bernie Sanders, and Sen. Elizabeth Warren would be the oldest presidents in history if elected, hypothetically taking office at ages 71, 78, and 79, respectively.
Karlyn Bowman at Forbes:
In questions asked in recent years, people appear somewhat more comfortable with the idea of a relatively young president than with the idea of a relatively old one. When a 2015 CBS News/New York Times poll asked registered voters what they thought the best age for a president, a plurality (46%) said in their 50s. (Most presidents have been inaugurated in their 50s.) Thirty-six percent chose younger ages (31% said 40s, 5% said 30s), and 10% chose older ages (9% said 60s, 1% said 70s). Eighty-three percent in 2015 NBC News/Wall Street Journal survey said they would be enthusiastic about or comfortable with a presidential candidate under the age of 50, compared to 63% who said they would feel that way about a candidate over the age of 65. In a new 2019 NBC News/Wall Street Journal poll, 58% of registered voters said they would be enthusiastic or comfortable with a candidate under age 40; 37% gave that response about a candidate over age 75.
A recent Economist/YouGov online survey told people that “if elected president in 2020, several candidates would be 80 years or older while president.” Forty percent said this might make it too difficult to do the work the presidency requires, while 34% said age helps candidates to have the experience and wisdom to do a good job. The preface to their next question was “If elected president in 2020, several candidates would begin his or her term before reaching 40 years old.” Forty-two percent said the candidate’s age would help him or her to have fresh ideas to do a good job, while 29% answered that a candidate being under 40 might make him or her too inexperienced to do the work the presidency requires.