Historically, Presidents typically tend to be in their mid 50s when they entered office. Kennedy remains the youngest President elected to office at 43 while Teddy Roosevelt is the youngest president at time of inauguration (Teddy was Vice President to McKinley, who was assassinated while in office, and Teddy ascended to the Presidency at the young age of 42.).
The reason for this is that Presidents are often the culmination of years of political experience, with service usually including either the Senate or Governor of a State prior to attempting candidacy. And typically, Leadership in either house in congress will be most senior and too stable to jump to the next level... additionally, some politicians are loved by the people who vote for them but are Nationally loathed. A recent poll found that the four members in congress known as "The Squad" who are all young progressive far-left democrats are supported enough in their distiricts that they could probably get re-elected, but nationally, their approval rating is as high as 25% and as low as 9%... they will not get Presidency with those numbers and would likely never try.
Additionally, there is a stigma against candidates who lost their last election from running for President, as they clearly do not have support, and many govenors will only run after their term limit is up.
Historically speaking, the 35 year old threshold mandated by the Constitution is the steepest, with the Representatives required to be 25 years old and Senators required to be 30 years old. Many of the founding fathers were in their late teens and early to mid 20s during the start of the Revolution, with Ben Franklin being the notable exception and being seen as the Cool (and Dirty) Old Man all the younger revolutionary thinkers hung out with as was George Washington (along with him being a Father to his Men as General of the Army). It's also important that the founders envisioned the Legislature to be the "first among equals" of the three branches with the Presidency being the guy seen by the world as representing America. Thus, the President needed to be a more revered figure and someone who's ideas could stand the test of time much more so than a young new shiny person entering as a baby faced Represetative. Sure, it's a popular idea now, but if it's popular in a decade, we'll talk about being the face of America to the world.
It should be pointed out that the 60+ candidates have only been a recent thing, as Trump is the oldest person to be elected President (70) in the history of the Union... and his 2016 opponent would have similarly set the record had she won. Going into 2020, most of the front runners of the nomination are also in their late 60s or early 70s and Bernie will be 79 two months shy of the election).
This may be due to Obama's extremely young age at election to office (46, not youngest but certainly in the top 10) and some of his detractors (and supporters if no one is around to hear them) point to his very short time in Washington as a factor that hurt many of his agenda goals' execution.
As a final say on the matter, while he is either a sainted hero or the devil's right hand man, most Americans love then ~72 years young Ronald Reagan's zinger when asked if age should be taken into account in the 1984 Presidential debate:
"I want you to know that also I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience."
If you find the clip, most won't shy away from a cut to his opponent, Walter Mondale, not even attempting to hide the fact he found the response funny and it sums up American's general attitude towards age: It's just a number and while it's discussed when out of the ordinary, it's ultimately no more likely to hurt as it is to help a President get elected.
