Statements above are based mostly on The Brain That Changes Itself by Norman Doidge which I highly reccomend. Plus just listen to some old folks that you can tell are still quick - Warren Buffet 86, Chomsky 88, James Harris Simons 79 and so on.
I didn't mean to show by these examples that this is the default, or even that it's common. I don't think it is. But I hope it's getting better, mostly thanks to the technological progress which can be stimulating for those who choose to make use of it.
But if you look at aggregate data on chess players' ratings as they get older, or university professors' publication rates (and citations on the papers they do publish), it's really hard even for strong-willed thinkers to stay on top of their craft in their 60s and beyond.
For every Warren Buffett in business, there are a lot of Sumner Redstones ... holding on to power, but quite erratic as age takes its toll. Getting old is nasty stuff.
Wow 3 of them! And it helps to ignore the (likelihood of) older people who kept using their brains and now have mush upstairs.