Biden's mother lived for 92 years, his father lived for 86 years. As far as I can tell there's no record of dementia in Biden's family history.
If Biden wins reelection and serves out his entire term, he will be 86 years old at the end of his presidency.
The right-wing attack on Biden as being too old and senile has been going on some time. Here's a politifact article debunking the claim:
A persistent Republican line of attack against Democratic presidential nominee Joe Biden is the allegation that he is no
www.politifact.com
What to make of all this?
I don't think that Biden is in mental decline. I do think that Biden is physically and mentally fit for office. However, Biden's family history suggests to me that age can't be completely dismissed as a concern.