I can answer the question of the title. I'm 59. I'm old.

I'm still working (university professor) and physically active (I ran a half marathon at a 7:35 pace two weeks ago), but yeah, I'm old.

My recall is worse; I'm past my athletic PRs; my curiosity about brand new things is less; my sense of possibility has shrunk within very narrow boundaries; I think more about endings than beginnings.

All of that does not preclude lots of joy and satisfaction in life, and productivity, and learning. But I realize that my orientation is "old," whatever my biological age is.