does that come from being biologically younger or just having had less time to become set in their ways?

I think humans are sacks of chemicals to a much greater degree than most of LW believes. As a simple example, note that injections of testosterone into older men tend to change their personality quite a bit.

I don't know if being less open to new experiences is purely a function of the underlying hardware, but it certainly is to a large extent a function of physiology, hormonal balance, etc.

fixable with better pharma technology

I hope you realize you're firmly in the "better living through chemistry" territory now.

if we can figure out how to temporarily put people into a more childlike exploratory open-to-experience state.

The idea of putting LSD into the public water supply is not a new one :-)