I think it's that "sense" we all seem to get somewhere near the age of 40-50; some get it earlier, some later; largely dependent on prior habits, choices and lifestyles, of course!
Funny, too, how that "middle-age group" definition changes its range as we get older. When I was in my 20s and 30s, I was sure middle-age didn't begin until at least the middle 50s.
Now, at mid 60s, I think I'm still in 'middle-age', since I've stretched the range out to about 75-80...or so.
So, what's all this have to do with smiling more?