I have never lived during a time when I was older than the US President However, throughout history, more than half of Presidents were younger than I am now when they began office.
I look forward to the day when I am older than the President. For a brief while this will be disconcerting in its unfamiliarity. On the one hand, this will also serve as a reminder that I am getting old — or, more to the point, that I will die, along with the entire world of my peers. On the other hand, it will be wonderful for the nation to be led by someone with recent perspectives (Obama is the only one who was born after 1946!) combined with sufficient life experience.
This will also mean that I have survived to be a certain age: an accomplishment of sorts.
A newspaper article from 1987 indicates that the life expectancy for a white male, in 1787 at the establishment of the Constitution, was 38 years. I do not know the primary source for this data. It does have the feel of being roughly correct, especially because the article specified that this was the life expectancy for white males. The life expectancy would have been different for women, who died more frequently in childbirth but not in war, and for non-whites. Probably the data was never taken for those populations. They literally did not count. The number is likely the life expectancy at birth; someone who survived infancy would have had a significantly higher life expectancy.
I don’t know if the Federalist Papers or any other surviving documents explain why the minimum age for the President is set at 35, which seems arbitrary to me. Some people speculate that this has to do with preventing immediate succession by a direct descendant: that is, a monarchy. Three Presidents intervened between John Adams and his son John Quincy Adams; in my own lifetime, the specter of monarchy reasserted itself, with only Bill Clinton between George H.W. Bush and George W. Bush.
I hope we never see a child of our current President in any national office.