I think the issue isn't so much that old people are close minded compared to young and middle-aged people, as that young people are very open-,minded compared to middle-aged and old people.

Indeed. It seems like our goal should be to optimize the level of open-mindedness in everyone, appropriately for their status in society and other considerations.

Also, an advantage of aging and death is that the people at the top of hierarchies get changed.

Also true, however this is only an advantage insofar as otherwise, without death, the people at the top of hierarchies don't get changed. It seems to me that our goal should be to avoid having hierarchies that work in such a fashion. Of course, that is a difficult project, but it's not obvious to me that it's an impossible one.