There is no objective answer to this question. It is impossible to determine whether people really change or whether they only appear to change because of a change in their circumstances. We cannot objectively measure what a person “is like” and then measure again to see if they have changed while controlling for all sorts of changes in their life circumstances. Therefore, any answer to this question is a matter of opinion. My opinion is that people do change, particularly over a long period of time.
As we age, we (in my opinion) can change in fundamental ways. In part, this may be because of physical changes in our bodies. A man in his mid-40s is likely to have less testosterone than a 20 year-old. The parts of his brain that allow him to control his impulses are more likely to have matured. Because of these things, he is less likely to be impulsive, rash, and aggressive. However, I would argue that these changes are partly due to the wisdom that he gains as he grows older. As we grow, we see that things that we thought were important when we were younger weren’t really that important. We see that we can find joy and pleasure in different things. This can make us less aggressive and more willing to take life as it comes.
We can also change (again, in my opinion) simply by wanting to change and by being influenced to do so. Imagine a person who is very self-centered as a young person. They get married and their spouse dislikes their self-centeredness. They see that their actions make their spouse unhappy. Over the years, they change gradually to become less self-centered. This is, to me, real change that comes about through personal effort, not just the appearance of change that comes from changed circumstances.
Of course, it is possible to argue that the changes I discuss here are simply caused by changed circumstances or that they are not true change. There is no way to prove that one way or the other. My view is that people can and do really change.