I don't mean that you forgot your age and then are surprised by it, but do you ever just think, "I can't believe that I am that old". This can occur to you at any age. I remember thinking that when I turned 23.
Now I am 66, I feel great, but really I have a hard time thinking I am that old. I remember clearly being in a race when I was 45 dueling with someone I later found out was 50. I was amazed a guy that old could still run well. Now 50 seems so young.
When I was 30 I would never had thought 66 could feel as good as it does. But still 66 is old, in 10 years I'll be 76! There are many cliches about age some of which ring true now. Time does have a way of sneaking up on you.
I am not at all unhappy with my age but, in some ways, I am surprised by it.