He's always been too old to run. It's completely ridiculous that he would go after his opponent for something that he is as well without a hint of irony.
He could shame others for having a pot on their head while he has multiple pots on his head and his voters wouldn't be able to see the farce.
It was always just about image. Trump has a bunch of other bad things to talk about and he doesn't show age as a problem as clearly as Biden does. Not to say there haven't been moments where Trump has had moments like Biden but the focus has always been on the other bad aspects of Trump. The reason they focus on it with Biden is because he doesn't really have much else interesting going on for the media to focus on instead.
Watch an interview of Trump in the 80s, after 9/11, and anything that gets put out now. A lot more controlled in the 80s, far more coherent train of thought in the 2000s, and neither of those things now.
He wasn't any less of a terrible person back then, but the veneer hadn't worn off.
One big concern is he has golfers spine. That's why it looks like he's wearing a diaper full of shit when standing. It's a serious concern as he may even need spinal surgery as bad as it looks. Has he even been checked by a spinal professional?
I would think with access to the best medical care on the planet, (for free even on our dime), he's been checked by someone competent. It's just that you can't fix everything. And at his age, it's probably not worth the risks that might be involved.
It's a preemptive strike. Haven't you noticed that when Trump has done something wrong, he immediately and viciously attacks his opponent for doing that thing. That's true even if the thing is being old. If you make a list of things Trump has accused others of, you'll notice quickly that he did many of those things. So many, in fact, that it will make you wonder if he's done those other things too.
He constantly accused Cruz of being a liar. He said Hilary was corrupt, racist, unstable, and that Russia wanted her to be president. He accused Obama and Biden of election tampering. It goes on and on. They say it's projection, but I think it's a calculated move to make the other people sound like they're saying "No, you are."