My dad is 4 years older than Biden. My mom is two years older than Biden. Other than the typical old people stuff (can't use technology, checked out on pop culture decades ago, etc.), there is little difference in how they are cognitively compared to 20-30+ years ago. Biden is a shadow of who he was just ten years ago. Granted, Biden has always been a gaff machine and likes to make up stuff. But he just can't express himself anymore and barely gets his point out. I don't think there is a max age to be president. People can be sharp as a tack well into their late 80s and early 90s. Bernie Sanders is as sharp as ever. Mike Wallace and Daniel Shor were as sharp in their 80s as in their 30s. Noam Chomsky is 95 and while being noticeably slower in his cadence, is still able to cite sources from memory and make complex arguments. But Biden just isn't there anymore. And he is getting worse with every year that goes by.