I felt older when our child announced his engagement, but not old.
I felt older when a parent passed away, but not old.
I felt older at the milestone of 30 years with my employer, but not old.
I think I will feel old when I can no longer do something independently that I more or less take for granted today...driving, walking, eating, remembering what I did yesterday. I can only hope that by then I'll feel that I lived my life fully and that I can find enjoyment and meaning by focusing on what I can still do.