[meowface]

It's a major fallacy to assume that evolution is somehow perfect. You might be right, but there could be hundreds of potential causes for the reduction in plasticity which do not actually involve protecting you from impairment.

As for your hypothesis, I think acceptance of norms and political beliefs is a process independent of neuroplasticity, which usually involves acquiring specific skills and talents; I might be wrong, but I think even a very "neuroplastic" elderly person may still have great difficulty changing political or cultural views.