I don't remember any particular age being difficult because of the age/number. Certainly, as Jean Luc once said "I've become aware that there are fewer days ahead than there are behind." Technically, that might not be the case, but at 45, I'm pretty sure that it is/will be.
The one year that stood out to me was when I was 37, and I became acutely aware that I'd been alive more years without the German Wall, than I had been with the wall. I don't know why that struck a chord with me, but it did.