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.
1: niceeee
Reagan was
So close to 78.