I think you have to distinguish physical condition from mental condition. I definitely do not feel as physically fit as I was when I was 40, or 30, or 21. I am reminded of this physical decline every time I stand up from my desk chair and must slowly straighten my back. That said, I still feel remarkably young from a mental perspective -- not 21 years old, but also not nearly as "mature" as I perceived people my age when I was younger. I suspect I will "grow-up" some day, but I am not in a hurry to do so.
That does make some sense. Unfortunately I have no way of knowing how I compare mentally to when I was 21 or 30 or even 40. I'm pretty sure I am slower and more conservative.