I too find this as puzzling (and irrational). Here are some reasons I can think of:
Just like buyers (incorrectly) perceive $10 to be much more expensive than $9.99, voters may (incorrectly) perceive an 81-year-old to be much older than a 77-year-old.
Biden doesn't bother dyeing his white hair. Trump does (blonde/orange).
- Biden's gaffes and stuttering
Doctors/medical scientists say it would be a sign of cognitive decline if one's rate of gaffes and stuttering has increased of late, which is not the case for Biden. (Biden's problem with gaffes and stuttering is a lifelong one.)
But this is a subtle point that most voters probably don't care about. When they do see Biden occasionally gaffeing and stuttering, they are quick to regard this as a sign of old age.
- Trump's brash personality
This probably reminds most voters of a dumb teenager.
In contrast, Biden's more measured style reminds most voters of the octogenarian statesman that he is.
Again, this is irrational (being fatter should make Trump's health a greater concern).
But I think to most voters, being fatter makes Trump look younger. (Survivorship bias means that there are few old fat persons or equivalently, fat persons are disproportionately young.)
Biden, being skinnier, looks more like a typical old man.
- Trump's orange skin color
Whether this is natural (as he'd probably claim) or not (as many have claimed), this makes him look younger. In contrast, Biden doesn't seem to bother much with make-up or other enhancements.
Olshansky, Barnes, and Willcox (The Hill, 2024-01-07):
Today his [Biden] chances of surviving through a second term in office are close to 75 percent (about 10 percent better survival than for an average man his age). Similar, although slightly less favorable survival prospects are present for Trump. ...
There is no clinical evidence for cognitive decline in President Biden — despite armchair gerontologists declaring otherwise. ...
It is also common to mistake Biden’s lifelong speech impediment for cognitive decline, his cough for an inability to speak clearly, his changed gate [sic] for a man in significant decline, an occasional misstatement as a sign of mental decay, etc. ...
President Biden exhibits characteristics consistent with superager status. The evidence so far — evaluated by experts in medicine and aging science and uninfluenced by politics and rapid judgments based on limited information — is that President Biden’s chronological age should not be an impediment to running for a second term in office.