I want to believe that you are as old as you feel but at some point you have to realize that you are not going to live forever even if you feel great.
What are the subtle signs of old age?

The bottom line is that you walk alone on your path

Maybe feeling tired a bit more than you used to, or another tiny line on your face that you did not notice in the mirror yesterday.
Whatever it is, there it will be, and you begin to accept it because there is no choice in the matter.
The biggest thing is your ability to have some control over you destiny. Can you still do everything you want to do?

Or do you have some limitations in your life?

I know that I have slowed down through the years and am not able to do everything I used to do but most of it is because of my mind; feeling trapped by fears that I did not have in my younger years. Today I want more control and predictability in my life and that may seem boring, but not to me. I try new challenges when I want to, they are not pushed on me, and that is good. And it is good to have friends in your later years to compare notes on about your feelings on your path in life; where you are going, what you are doing, and how it is affecting you and the people closest to you.

And in the later years you will change; not just your body, but your mind and soul. Some of it is good and some not so good feelings will surface that you did not have in your younger years like realizing that life has a beginning, middle and endings. And facing the endings is critical to your well being because you do not want to die depressed or angry; you want to be remembered with a smile.

Develop some healing ways that make you feel good and rub off on others. Do some things on your own like developing a new skill, and do some things with others like playing golf.

And always live today like it is your last day on this earth, finding ways to be kind to others, smiling at a stranger, giving the gift of a warm embrace to someone you love.