I think that the closed-mindedness of elderly people is more likely cultural than a biological fact of humanity. While the cliche runs that science progresses one death at a time, in my experience, old scientists usually have discarded and continue to discard great numbers of once-popular ideas. Science as a process gives people a mechanism for rejecting old ideas, and on the whole it's pretty effective.

Lacking effective mechanisms for changing their mind, people in general do not need to get old in order to become closed-minded.