Then your conclusion that people over thirty do not change their minds is based on false premis!
There's a difference between "many" and "for all".
Over the decades I've had a number of offline intellectual sparring "partners", where we would argue various issues from different perspectives. In these heated discussions, we mutually agree to not dig in our heels too much. But as we got older, I've found that it was harder to convince the other person of a particular point, regardless of the supporting/dissenting evidence.
Whether consciously or unconsciously, we find ourselves digging in our heels more and more as we got older, than when we were younger. We frequently joke that our minds are "calcifying".