Mental decline with age is a thing, and in my personal experience, it starts right at the boundary of the forties.

All said, if I had as little as 200k in my bank account, I would start investing less time on getting more money and more time and money on getting more health. From my very personal point of view, a person who has millions on their name and who is still working for more money has a bit of a death wish.

63 is young; most my current business partners are over 50-60 and a few of them are over 70-80s and all of them work full time. They are all millionaires for a long time already but they like work and enjoy building things. So do I; I don't think I will quit before I die or until my brain indeed doesn't work anymore. The 70+ guys are not quick with some things, but have tons of experience which makes them better/quicker in other stuff which matters far more to the business than tech, speed or 'good hip ideas'.

It seems to me that personal finances should take a lot less time for someone who has a billion dollars vs. someone who has, say, $100k.

The latter can lose it all on a bad financial decision, and probably has ideas for what they could do if only they could double or 10x or 100x the money, so risks can be worthwhile.

The billionaire will keep getting richer beyond human comprehension even if they just put the entire fortune into treasury bonds. 4% interest on $1B is objectively so much money that you shouldn't bother trying to get more if there's anything else in your life that you'd rather spend time on. (Of course a lot of billionaires get there because the single one thing they want in life is to make money and see the number grow, but Cook doesn't seem that way.)