He wasn't exactly eager to share the information, although I'm give him credit for acknowledging it when the Times found out about it.
And it wasn't the only medical issue that caused some brain damage. The article also mentions mercury poisoning.
Biden seems older. Trump seems crazy, but that doesn't read as old. It's possible that there's room to convince people that Trump is suffering from age-related cognitive decline, although that may not be a discussion the Biden campaign wants.
Voters are looking for different things from them.
The comments about Biden's age allow for the argument that others are making the big decisions, so that even if a voter likes Biden as a moderate-ish Democrat, others who are to his left are really running the country.
Many Trump supporters like him enough that they'll consider his age to be worth the tradeoffs. Because if he didn't run, it would be a typical Republican anyway and that's what you'd get if he had to resign for his veep (assuming it's someone like Stefanik, Britt, Huckabee Sanders, Tim Scott, Rubio or Burgum.)
For Republicans who will vote for Trump because he's the party's nominee, they like the idea that some establishment Republicans will be running things behind the scenes.
There could be a potential argument that if Trump is declining, it's an opportunity for grifters who wouldn't be allowed near a presidential administration.
There's some pushback against the question, but there is a reality that Hamas is not getting the message that everyone in the civilized world sees them as a force for evil on par with the Nazis (in terms of what they want to do, and what they're doing when they can.) Too many of Israel's critics are playing footsie with vile antisemites and terrorists, and there should be the same criticism against that as there would be against conservatives who aren't as eager to bash white nationalists.