My personal theory is that it's all about ensuring continued personal and professional growth. I know plenty of people who stoped those in their 20s and seem "old". I also know plenty of people in their 60s and 70s who didn't stop them and I find myself surprised regularly when I remember the number of years they've been alive.
Basically just supporting digitalsushi's point with anecdotal evidence.
In tech, you constantly need to keep up to date on the latest technologies, read publications and go to conferences, because experience gets stale very fast. It's also easier (than in fields like medicine or law) for junior techies to get up to speed in experience, because they can skip the decades of now obsolete tech and fast forward to roughly where you are at. This is fine. It can be fun and it's definitely rewarding.
Then by your early thirties, three things change at the same time: you get promoted to a busier, more responsible position, you have small kids, and you get fat. The (waking) time you have for yourself shrinks from 8-10 hrs to about an hour if you're lucky. You can use that hour to study and keep up to date, to do something about that gut and exercise, or you're so tired you just want to relax. Pick one.
In my opinion, this is why middle aged people seem so lazy and boring.
And there's other things, like how at 30 I had totally lost the ability to squat down unaided - I had to prop myself up with a hand, and now I like to drop down and sit in squat position just because I enjoy the feeling of my newfound strength and mobility... I don't dare think about how drained I'd be from just short amounts of playtime with my son if I didn't exercise regularly.
Exercise is an answer to too little time, not a problem.
The big difference is for the last 8 years I've put in 3-5 hours of hard exercise (powerlifting) every week before work. I wish I had more time, both for exercise and projects, but these days I can pull all nighters and walk into the gym 7am the following morning and set new personal records a couple of times a week, and keep my energy up all day, whereas at 30 I was worn down to the point where I couldn't walk up a flight of stairs without my knees hurting, and would collapse on the sofa in the evening and not have energy for anything.
Now I am in better shape than most of the kids half my age at my gym, and have early 20's body builders talking about how deep I can squat.
And I go home after work, play with my son, and put in time on my projects after he's in bed.
Sure, I had downtime when my son was smaller, but kids are not an excuse for all that long - when he was younger, I put projects aside and my only "me time" was focused on keeping fit, and I feel it paid off. Now, every day, he can do more stuff for himself, and wants to, and every new achievement means more energy left over for me.
Why? Better diet (Paleo) and daily exercise. Bodyweight and Pilates, in my case, but I'm planning to try powerlifting at some point. (I remain convinced that mind-body exercise like T'ai Chi, Pilates or yoga are one of the best body hacks out there. )
For some additional "Old Guy" inspiration, here's Joe Pilates, the inventor of Pilates, at 51 and 82. He, erm, doesn't exactly look it: http://www.vivianzapanta.com/look-at-joseph-pilates-age-51-a...
People sort into two bins: Those who make choices, and those who make excuses.
I think it's often the people who think that they should have already won the prize that end up in the latter category. So they spend their time rationalizing why they haven't gotten there, or what is holding them back.
I've realized that there's no prize to win, there's no time limit, and that I can learn new things far more quickly than I could when I was younger. Instead of making excuses, or living in awe of past accomplishments, I just do new things. I take new notes. I learn new languages. I got a keyboard last month, and I started studying music theory and writing some compositions, and I play along with my baby on my knee, trying to play in whatever key it sounds like she's banging her hands on.
Some people complain that chores are hard with a baby. I challenge myself to do the chores one-handed with her held on my hip when she needs to be held. She loves the experience, I get extra exercise, and the chores get done, and I spend time with her, and it's kind of fun to figure out how to do certain things that you expect to have 2 hands for with only one.
I'm no superman, and I don't try to be. I do get to slow down, but I do it without regret.
I try to spend the time that other people take regretting the things that they can't or didn't do to do the things I want to do.
The morning is the best time to take care of the body. I am a cyclist and I love nothing more than to wake up early (well, somewhat early) and getting in an hour-of-power before breakfast, coffee, and work. I start the day off right and by the end of the day, I feel naturally tired instead of run down (there is a difference).
I'm not saying exercise is bad, and I do try and keep up with it, but am I the only one that has to think, "I can't exercise now or else I'll be too tired to code later."?
The right exercise regiment is important. There's no need to push yourself hard the first three months if you're starting out. Try a couch to 5k run routine over six months.
In the world where App Academy and the like are churning out iPhone developers, Ruby on Rails programmers, and Javascript (uh, what's the word for person who works in JS?.... ) advocates in eight to twelve weeks doesn't make those sound like skills to hone to get your long-term stability.
Personally, I keep being surprised by how much value there is in just remembering how tech worked in the 1990s and even the early 2000s - Windows and OSX and Linux have changed a lot and iOS and Android seem completely new - but a lot of platform decisions evolved gradually in the changing technical context, and throwback behaviors can seem completely mysterious to someone who's first programming environment was XCode - there's still DOS behaviors hidden in some of these places! I wouldn't be surprised if ye olde mainframe experts had some even subtler knowledge about how-things-really-work-under-there than I do.