I never experienced any 'oh shit' moments, but I could definitely tell things were going south when I got into my 40s. I used to always be 'Mr Ready' as soon as it was on, and there is something great about giving the wife the confidence she's still attractive by unleashing a beast not a wiener. 47 now and there have been occasions when 'the snake was not awake' until prodded. Here's hoping.
I have to say too, all the NO stuff is brand new to me, but today after a bag of Arugala (we call it Rocket, no really), 10 grams of beetroot powder, 3 grams of arginine and 3 grams of citrulline, I had warm ears and veins on my forearms. Let's just say there were other interesting developments too.