Actually the smartest person and the most flexible mind I know is also the oldest man (87 y/o) I know. It's infeasible for younger people to have his outlook, that simply comes with his life experience.
He volunteers weekends removing trash from the public parks and cleaning up our streets of litter.
I am teaching him how to play professional chess and we had plans to go to professional tournaments (but covid hit so maybe next year) and he is picking up really well.
What if: Neuroplasticity allows you to build new base cognitive tools from sensory information, or dump/change existing tools. If you build sufficiently numerous, broad, and adaptive tools when you are young, you will be able to do many, many things when you are older.
Still might have challenges learning new languages, or say to dance if you never learned how to dance at all, at the same rate as a toddler, tho.
Regardless, if we figure out how to live more and end up with brain zombies, we'll also have an incentive to figure out how to improve our brains. There is a lot of research into such topics too (see topics on research on Alzheimer's disease which are also posted here often).
It's not only about flexibility. When eventually Rupert Murdoch dies, his wealth and power will be split up amongst his children and the state. But if he lived on and on we'd be stuck with him forever.