I can't be XX years old. I'm feeling (XX minus 10) years old.

The article "Why Your Perception Of ‘Old’ Changes As You Age" (here)

I guess it is a truism about how our perception of age changes as WE OURSELVES age.

I remember 69 years old sounded old.  Now, that "sounds" young.

EXAMPLE: "My god, David Bowie was only 69.  That's so young."

...honestly, he was really much too young.