You are correct that accepting one's age has, for many of us, a negative connotation. But, that only emphasizes my point... why has a word, in this case "accept" or "acceptance," taken on such a stigma? Only because "acceptance" in this context has come to connote "resignation, "surrender," "giving up." Whereas acceptance in most other contexts is a virtue, something to strive for.

I guess it's somewhat similar to skin color. A black or brown person is wise to accept the fact that their skin color affects how they are perceived by the world, as well as how they look at themselves. Their racial heritage is something they cannot change. They learn to take extra precautions in certain circumstances. They don't necessarily "accept" prejudice but they should be aware of attitudes, threats, and systems that hold them back. What one does about it, how one thinks about it is up to the individual.

Old is a fact, a number. It shouldn't be a state of mind. Still, as you say, we certainly should be cognizant of what our aging bodies are capable of and adjust our behaviors and expectations accordingly. Not accepting one's age means a person is living in denial of reality. The challenge, in my opinion, is to accept the number and the wear and tear that comes with it, and still keep a positive, constructive mindset.