When do we get old

I have lived as free and as fast as I could for most of my life. Even since I met Mary, I play with the cars and live like a single guy with all sorts of toys to play with.
Even today, I don't even think that I am old until I see a reflection in a window or a mirror.
I have no daily medications to take. I have decent energy. No daily aches or pains either.
When I turned 50, the nagging thought of mortality crept in.
I'd already lost both Parents, both Brothers and all my Aunts and Uncles so death was no longer something that just happened to people that I barely knew.
I don't dwell on the end, I just think about it more than I want to. Before, it rarely occurred to me that my time was limited.