Its not the number of years, its the health of the brain. There have been some people in their 90s that are sharp as a tack (60 minutes once had a special on some WWII translators/spies, and they had a few people in their 90s - at least one was still an active College professor - who came across as extremely sharp people).
Biden's issue is, yes, he has always been dumb, and a sociopathic liar, but without a doubt his brain has crumbled the past few years. He wasn't able to campaign in 2020, and he can barely be trotted out to read from a teleprompter nowadays. I'm sure that his handlers have him hyped up on drugs, but, as I understand it, drugs can provide brief moments of lucidity, but over time they accelerate the aging of the brain. I'm sure that Biden has been taking these drugs these past few years, and at this point his brain is completely shot.
Its not the number of years, its the health of the brain. And Biden's brain is mush.