I think that there is something to a fixed personality that solidifies around age 20. This age probably corresponds to the end of the period of the greatest growth of learning in most people.

Perhaps that is a poor choice of words. What I really mean is that, for a majority of people, frequent exposure to new and different people, ideas, and things comes to an end after high school or junior college. Then comes the lifelong rut that you mentioned. It is very difficult for personality change when there are few stimuli to elicit change. Thus, personality becomes fixed.

But it isn't always this way. Many people continue to be exposed to all these varying stimuli - thus the opportunity for personality to vary somewhat over time. But I still believe that there is a "core" personality that does constrain the quantity of change and the direction that that change takes.