“Spirit” is pretty hard to quantify. In terms of cognitive ability (which seems to be what you’re referring to when you say “thought), age takes it’s toll. But then you speak of our “essence as a personage”, and I don’t see that as being a matter of cognitive ability, really.
I see some people get more and more set in their ways as they get older. Their worldviews become fixed, they live more in their memories and become less open to novelty, and take refuge in their habits. You could say that their person-hood has become stronger, in that it’s more resistant to change or growth; they become “a character” who stands out all the more for having stopped moving along with the world.
For others, the reverse seems to happen. The hubris and idealism of youth yields to the softer embrace of ambiguity and practicality. Connection, both to family and friends and to the world at large, supersedes former concerns about getting ahead and standing out. Positions mellow as experience reveals the complexities of reality. Someone like this remains engaged with the world because they don’t differentiate themselves from it. It may seem like they’re less uniquely their own person because they’ve remained flexible and flow with the current.
A good question, then, might be are we closer to our “essence” when we differentiate, or when we connect?