This is based on their age at the beginning of their term.
Trump was 70 at the beginning of his term and 74 at the end. If he is elected in the upcoming election, he will be 78 when he starts again. If he serves a 4-year term only, then he will be 82 at the end of it.
Joe Biden was 78 when he started. At the end of this term, he will be 82. If he is re-elected and completes a full term, he will be 86 when he finishes.
Thus, if Trump wins the upcoming election and serves only 4 years, he will be the oldest president by 159 days over Joe Biden.
If we rank by the oldest age they reached while in office, Ronald Reagan would be in 2nd place since he served from 691 to 77 years of age.
Interestingly, the 4th oldest president was William Henry Harrison died 31 days into his term in 1841, which is the shortest presidential term in US history. The prevailing hypothesis on the cause of his death was pneumonia followed by possible typhoid fever after giving the longest inaugural speech in US presidential history in freezing cold temperatures.
He was too old. We had younger choices then. Now we don't.
Many voters don't understand that the time to choose younger candidates is at the beginning, not at the end (in the general election). Most of these people complaining about no choices are not planning on volunteering for or donating to any candidates at all.
They're not making the pizza or paying for it, but they definitely want to choose the toppings.
Pre 2020 the oldest person to start their first term was 69.
trump beat it by being 70, then Biden blew that record out of the water, being 78.
It's not a good trend...
And we were actually doing really good until it was Hilary's "turn". Bill and Obama were both (relatively) young, and I don't think even James Carville is dumb enough to say their (relative) youth wasn't a big reason for their success.
Hell, I still remember Bill Clinton on MTV, even tho I knew nothing about politics back then.
They might not have been as progressive as they acted during their campaigns, but at least they actually tried to get the youth vote.