Do people really change?

So I posted something on my twitter the other day stating "People don't change, situations do" and I got mixed reviews about the topic. I believe that once people reach a certain level of maturity and intelligence that it is basically impossible for them to change. I don't think that a person that has been set in their ways for too long can change themselves. And with the changing of situations I believe that any situation can be change. I believe that a person just has to be smart enough to realize whats worth changing and what you just need to walk away from. And with that said I have walked away from a bunch of situations this year and I don't regret any of my decisions.