When I was 18, I wasn't one of those youthful idiots who considered 40 to be 'old'. I did consider it to be middle-aged. Back then, I thought someone was 'old' when they were 60. Truthfully.
When I was in my mid-30's I started to redefine 'old' at around 70.
Now that I am over 45 (and minimum full retirement age is defined as 67) I still think 'old' is around 70, but more likely around 75.
Why the definition shift? Is it purely my own closure proximity to my original definition of 'old' (60)? I don't think so. I think it boils down to the age of my friends and others whom I am close to.
I have several friends who are in their early 60's. My in-laws are in their late 70's and they are quite active. How can I have friends and loved-ones who are 'old'?
Grendel cracked me up recently. I showed him a current picture of my father, who is in his mid-70's. Grendel looked at the picture and said, "Wow, he's old." I didn't bother pointing out to him that my father is the same age as his other grandparents, whom he visits with on a regular basis. I couldn't bear to hear him say that they were old too; though I wonder if he would really say that since they play tennis with him and go boating with him and are generally quite active.
I sometimes wonder if Grendel secretly thinks that his own parents are 'old'. I didn't start thinking of my mother as old until she was in her mid-60's, when she started kind of showing the initial signs of decrepitude.
Maybe my definition can be explained by the fact that none of my older friends and loved ones are showing signs of decrepitude, so I can't possibly think of them as 'old'.
Can you be 'old' without having signs of decline? I guess I hope so. I wouldn't mind getting 'old' as long as physical decline doesn't have to be part of the bargain.