I'm thinking of things like... nose hair. You've got nose hairs. You've blown them out occasionally when you blow your nose. No-one tells you they're going to defy your best attempts at shaving and make a hitler moustache all by themselves. Once you're past the big three-zero, you'll need to invest in a certain cylindrical battery operated device, or get yourself a pair of tweezers.
I'm thinking of things like... you know how you used to be able to run down the street and barely get out of breath? No more, my friend. You can still run down the street, but you'll get a few houses down and your legs will start to stiffen up. But the good news is, once you've run out of puff and you're having to rely on your wheezing breaths to keep you going, you're about as fast as you've ever been. Just you don't get so much of a free head start as you did when you were younger.
So for the benefit of our bum-fluff faced brethern, what have you discovered as you get older that they never told you about in advance? Both stuff that was a surprise, and stuff you were dreading that actually hasn't turned out as bad.
Cubby didn't know how to turn off sigs!