Eh, he's always been eccentric a-f and made those gaffes. He's never been any different. In the '80s he'd confuse his Atlantic City NJ casinos with his ones in Vegas when speaking. Non-issues.
Biden's age is overrated, too... Kissinger, like him or not, was all there all the way until he died at 100. Plenty of examples. Chuck Grassley is a plenty competent senator. Age is just a number. Competence and senility is indeed positively correlated with age, but not necessarily caused by it. There should never be age limits or age discrimination because age is truly just a number.
Heck, Jimmy Carter was a poor president, but he was also a one-term president so technically he could have come back and run for a second term in all of these years, and he was always very lucid well into his 90s until very recently.