Since my son did become much more quiet and to himself when older, I've had to sit back and just watch. I don't want to hold unfair expectations for him, but I do want to make sure he's happy. Sometimes changes can pinpoint something is wrong (like with some kind of psychiatric issue, like depression, etc.) or it can simply be situational, or maybe the person is just exploring new parts of themselves. I see his extroversion most nowadays when he's with his friends, and the fact that he still has many many friends and is very popular among them. It was interesting too to note that when he got "quieter," it wasn't all a bad thing; he actually is much easier to hold an evolving conversation with, and he can go much more deeply into a topic than he used to when he was much more surface-oriented and bouncing from one thing to the next. He's not nearly as emotionally volatile as before either. I love that part of his "deepening."
We just keep changing, don't we? And we're much more textured and complex than the theories used to describe us.