One thing that has generally remained the case throughout US history is that US Presidents tend to be a bit older. This is not surprising, given that most US Presidents serve in government for some time before running for president. In any case, there are some exceptions to this rule.
The youngest US president is actually Theodore Roosevelt, who became president at age 42 following William McKinley's assassination. However, in terms of the youngest US president who was actually elected to the office, it was John F. Kennedy. John F. Kennedy became the President of the United States at age 43.
There have been other US presidents who were elected to office at a relatively young age. Another one of the younger presidents is Bill Clinton. Bill Clinton took office as the President of the United States at age 46. This makes him the third-youngest president in the entirety of US history, behind only Theodore Roosevelt and John F. Kennedy.
Ulysses S. Grant was another US president who took office at a relatively young age. Grant was a hero of the American Civil War and that was one of the reasons he was able to win the 1868 US presidential election. In doing so, he became the 18th President of the United States. Grant was only 46 years old when he took office and he was actually only 82 days older than Bill Clinton when he did so.
Of course, there are more recent examples of relatively young US presidents, as well. One notable example is Barack Obama. Obama took office as the 44th US President when he was only 47 years old. As a result of this, Obama is the fifth-youngest U.S. president in the nation's history.
The next president on the list of youngest presidents is Grover Cleveland. It is important to point out that Grover Cleveland actually served two non-consecutive terms as President of the United States. Cleveland was also only 47 years old when he took office as president for the first time in 1885. It is also worth noting that Grover Cleveland is the only president in United States history to serve two non-consecutive terms.
The seventh-youngest president in all of US history is Franklin Pierce. Pierce was 48 years old when he took office as president. Pierce won the 1852 US presidential election and became the 14th President of the United States. It is notable that Pierce began his political career when he was only 24 years old.
As you have likely noticed, there have been US presidents in the nation's history under the age of 40. The U.S. Constitution does in fact set an age requirement for U.S. presidents. According to the U.S. Constitution, the President of the United States must be at least 35 years of age or older. This requirement is relevant to note when discussing the youngest presidents in United States history.