from my perspective, the reason to consider age limits for politicians, while using Biden as an example, is not the chance that he will die in office; it's rather the chance that he won't die in office.
he's showing signs of having trouble doing a good job
As if he didn't show those signs when he took office.
Presidential age limit restricts mostly the right of the people for who to vote for. If you can't trust your people to distinguish candidates abilities to perform their duties, maybe a different political system altogether would be more appropriate.
Now, when media start covering up, maybe the problem also lies elsewhere
The electoral college picks. They don't have to vote for who most the people their state wants either (although state law may punish them if they go rogue).
President is only semi democratically decided as a courtesy.
he's showing signs of having trouble doing a good job