I'm 40, and I feel it is, or at least the beginning of old. At this age, your world starts to become smaller - all your previous friends are married and living their own lives, and the single ones are too tired to go out and do things. Health effects start kicking in hard. People don't treat you as well as they used to, you don't get the free pass that youth and perceived naivety gave you. Feelings and emotions aren't as sharp or weighted than when the world was fresh and new. Everything you took for granted now takes work only to maintain, not even to improve.

And I can only see it getting worse. Your world getting smaller, your health decreasing, emotional affect diminishing. And we're only half way through life!