From what i understood by other articles posted here at the past, unless there is some actual brain issue, the practical difference is very small and it is largely a matter of self-imposed limitation (e.g. people stop learning or get set into their ideas because that is the path of least resistance, but if are placed in an environment that encourages or forces them to learn they can do that fine).

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).

perhaps flexibility of thinking is not about neuroplasticity, but instead having many pools of thought and experience to draw from.