Legally, anyway, you’ve been an adult now longer than you were a kid.
It’s all downhill from there, bunky! 
For me it was the change of my metabolism. Up until I was about 45 or so I could eat whatever I wanted and hover around 175 regardless if I exercised or not (I’m 6’3). I’m turning 60 in 2 weeks and if I ate like that now I’d easily weigh 300 pounds. I have to watch my diet now and work out a few times a week just to keep under 220.
Actually, one surprise for me is that i can’t eat all i want anymore. I don’t mean, “if i eat everything i want I’ll gain weight”, i mean, my body says, “you’re full, stop eating”.
Sometimes that happens when I really want to eat more. 
Yes, I’m already experiencing this and I’m barely over 50.
I wish I would’ve known that days without pain would disappear. I’ve had a lot of injuries in my lifetime so I’ve had pain. But, in between those injuries, I would have days without pain. You don’t think much about it until you realize you never have days without pain and you most likely never will again in your life. My neck started hurting the end of last year. MRI showed a bulging disc but they claim it is most likely arthritis. I haven’t had a day since that my neck hasn’t hurt. My knees also hurt everyday (fluid in one, arthritis in both). My back hurts often. Getting sleep is problematic on some nights due to the pain. All of this and I am relatively healthy. Not overweight, I bicycle, hike and walk a lot. 
My fellow gentlemen with the drippy pee thing … has no one told you about “The Button”?
I’m about to change your lives; ladies, you’re on your own; you’ve got your own mystery button to worry about.
The Button. Sometimes referred to as “The Taint.” What you do is, when you’re done peeing and you’re trying to wring those last few drops out of your weiner, you reach under the boys, you get a big fat finger right where the taint sort of covers the prostate … if you’re following me here. Give that fucker a good stiff press and voila … it’s like and Air Jordan pump for pee.
You’re welcome.
72 here, loss of muscle, man boobs. In most ways holding up better than I expected considering how badly I had treated my body.
A few different things:
I had really bad acne as a teenager. My parents kept reassuring me, “Once you get past your teens, it’ll clear up.” Nooooope. While it got a bit better by the time I got to college, and continued to decrease in severity in the years that followed, it wasn’t until I was well into my 30s before I got past having regular outbreaks.
I did finally calm down, in general. While I wasn’t hyperactive as a kid or as a young adult, I could be overly energetic and excitable, and I had a bad temper. I kind of assumed that that was just who I was (and I hated the fact that I had such a bad temper). But, at some point in my 40s, I realized that my temper was finally starting to mellow, and I wasn’t as “always-on, high-energy” as I had been as a younger adult. And, now, at 55, I’m at the point where I might lose my temper a handful of times a year – and always at something stupid, like my computer not working properly.
Don’t forget that time you were snappish with me.
[/feeble attempt at humor]
Can you ever forgive me? 
Of course…thanks for coming along for the little snicker.
I remember going to the doctor a few years ago for my annual checkup, and rattling off a handful of things that had been bothering me. He just kind of chuckled and said “Yeah, getting old sucks, doesn’t it?”
Then he stuck his finger up my butt.
I’ve had the same doc for 25 years, and he’s well aware of my aversion to that procedure. A few years ago I was bent over and, as usual, he grabbed my shoulder to keep me from getting away. I told him how much I hated that moment where one hand was holding my shoulder and he said: “Don’t worry unless you feel both hands up there”.
Like you, I groused to him about the growing list of aches and ailments as I aged. He said: “Your body’s designed to last about 40 years. After that we’re not getting you well, we’re just managing the decline”.
The first doctor I had after I stopped going to a pediatrician said, “please don’t act like you enjoy this.” He had fingers the size of broom handles. My last two doctors have been women, and I was glad that they were able to do the procedure with one finger.
I just turned 60 this year. It blows me away that I reached it because I was suicidally depressed during more than one period of my life. Part of me was just bent on getting through the depression, and it seems that part won. Yay, me. Other surprises: I feel that I got here awfully fast; the longer you live, the faster time goes. I’m surprised that I look younger than my mother did at this age. She was thinner, but I don’t smoke. Part of my brain still thinks I can do anything. Arthritis and a newly slipped disk tell me otherwise. The big surprise for me is really how scary the current times are in the U.S. 40 years ago, I thought that maybe I’d be looking at a retirement career of volunteerism. Nope, I must work instead because as my body falls apart, so I must pay more and more to keep body and soul together.
But I’m still here.
I was fine with getting older until about five years ago, when they decided that the chest pains I’d been having for twenty years actually had a cause, and they installed two stents. My body apparently took this as a sign that my warranty had expired, and since then I’ve developed liver problems and anemia. My knees also joined the party, and the intermittent pains over the past forty years are now pretty much a daily thing.
Nice advice, Jack, but where am I gonna sprout this third hand? 'Cause both of mine are gonna be full.
I’m 41.
Good: I can take afternoon naps now. Like, if I’m just at home and there’s nothing urgent happening, I can pretty easily fall asleep; and it’s a very sweet sleep. Then after an hour, say, I naturally wake up and can continue whatever I was doing.
Neutral: I agree with other posters that it seems to have happened really fast. I regularly forget and sometimes think of people in their 30s as older than me.
Bad: Difficulty getting up from a seated position. It doesn’t make sense to me, because I work out; I can do chin ups and dips no problem. But, regardless, it seems I have to psyche myself up just to stand up now. It’s weird.
Not saying this is the worst thing about ageing, just answering the OP about unexpected things…
Well … sounds like you may not have a hard time finding an assistant.
My father (now approaching 80) noted that decline in older folks like himself tends to be almost like a descending series of plateaus. You get sick/injured and recover - but you don’t completely recover. You just mostly recover and find a new equilibrium. Sometimes slightly lower than before, sometimes a fair bit lower. And then you proceed along like that until the next significant injury/illness/medical procedure and you drop down another tier. And the older you get the more often health issues arise and you begin to spiral down more rapidly. In stages.
He was also the one who disabused me of some of the joys of retirement. I was remarking on how much time he and my step-mother now had to spend on hobbies and he noted they had all the time in the world, but only a fraction of the energy they used to have. They are pretty much reduced to having the energy to do ONE thing a day.
Meeting a friend for lunch? Fine, but that’s all you are accomplishing that day. Big Costco stock-up trip planned? Whelp, there’s another day. Oh, a medical appointment in the morning? Day shot.
It’s really why you want to retire sooner than later if you can swing it.
I saw a similar thing with my parents. But I know they economized, too—maybe sometimes it’s easier to say they don’t have the energy vs. they don’t want to spend the money? Funny thing is, I was always waiting for the call that said we kids had to pitch in for parental care. That call never came. In fact, dad died 15+ years ago and when mom died, we all got a nice check from leftover assets. How the hell did a couple of blue collar workers manage that? Answer: they were Depression-era folk; they knew how to not spend money.
Yeah, you better accomplish some things now. The energy and money are not assured, even if the time becomes plentiful. Besides, you’ll probably enjoy the experience more when your knees aren’t complaining etc.
Yeah, that drives my mother nuts. And when she’s not doing well, she may only have the energy to do something every other day. 