At what point did you realise you were "old"

when i was about 48 ish . I am great health today though. I work out 3-5 times a week have great cardio high energy but small things I need to change. For instance I bike instead of run to preserve my knees. I moved my diet to a higher concentration of veggies and removed most sweets. I shifted in my hobbies a bit to accommodate my age. I read a ton when I was young but now I am slowing down in reading because I have a few hundred books I have already read and don't feel the need to keep that pace anymore.