I used to say "age is just a number". I don't remember when I last said that.
Now that I'm in my 40s, "age is not just a number". My mind has aged pretty rapidly than I thought it ever would. Approaching midlife, I can feel the heaviness of my "numbers" in my body and mind. It could be just me who nearly abandoned my health, but I presume same is true for most of the people in their 40s and onwards.