What has recently made you realise you're old?

I recently went for a meal, on a Friday afternoon with my fella. We parked in a pay and display opposite the restaurant. Rather than walk slightly across the car park to the pavement, I decided to take a more direct route of walking towards the restaurant, which involved a small wall to step down from to get to the pavement. No more than 1.5ft of wall if that, it was level to the carpark so it was in fact more of a large step down. In my youth I would have done this without thinking, but at 43(in December) I evaluated it, tried to do it with caution and jumped/stepped in such a way that I not only jolted my back but both knees too. Wtf !! I'm not decrepit, I'm no athlete but I can move. This wasn't just a random thing, I feel this was the start...of something. Anyone else relate?