The most common is a slow hardening of one’s opinions and perceptions that I consider a form of mental decline. These people are often susceptible to propaganda and conspiracy.
But I also know people who’s curiosity expands and their thirst for knowledge grows with a deep understanding of life informed by experience. You can have wonderful conversations full of nuance that are inspiring.
You can guess which is more interesting and rewarding.
Sometime ago I read a popular article that compared artists and scientists who peak early in their careers to those who peak much, much later - building on one hard earned success after the other. I wish I could find the link because my summary here is probably missing the mark a little.