“Giving up” childish shit? Hell, I’m in a position now where I can adopt more! 
I’d like to think it’s less that we’re doing childish things now as that we must have been doing exceptionally mature things as a kid. If you have fun doing something then age shouldn’t even be a consideration.
I’ve been doing the same things for fun (and profit) for well over 20 years now, and I see no reason to change. My life is fun.
I’m still the only person I know who is exactly the person they set out to be.
What do you do?
That’s me.
My personal tag line: I’d rather be doing something than watching something.
When I was in my 40’s two friends told me that I should quit downhill skiing. They said “skiing is for kids, act your age and give it up before you get hurt”.
I’m 65 now, and still skiing… the two friends who told me to quite skiing are both dead.
I still hike, roller blade CC & downhill ski. My skiing has slowed down a bit, no more jumps, I seldom do a double diamond run, and I avoid busy weekends and late afternoon skiing.
You don’t stop playing because you get old, you get old because you stop playing.
You can have my tabletop war-gaming figures when you pry them from my cold, dead, paint-stained fingers.
I was singing along to Paul & Storm… or it might have been Jonathan Coulton in the car the other car – heading to a comic and media con with my 9 year old (he wanted to go too… really…) and when he confirmed that he thought his Dad was weird (the rolley eyes are the clue…) I told him that his Dad may have got older, but he hadn’t grown up! (And has little intention of doing so).
Um, very reluctant. I’m 23, but within the last year have realized that this means I’m past the stage of peers judging me for what I do and I have the freedom to do little kid stuff if I want. I just bought Harry Potter Legos for myself. And I still like the Hardy Boys. What’s the fun in living if you can’t indulge yourself?
For a living, I’m a theatrical production electrician: I spec and build lighting rigs for concerts, theatre, tradeshows, TV, movies, etc. There’s only a couple hundred or so people in the country who do what I do for a living, at most, so I’m fairly well paid by clients and I get to travel frequently to exotic destinations like Detroit, MI and Dana Point, CA. 
Plus, sometimes I’m not the guy in charge, one of my friends is, so I get to work without having to be responsible every once in a while. It’s a good deal to just be a wrench some days, ya know what I’m sayin’?
For fun, I goof off, play video games, play guitar, goof off, play some more guitar, read books, listen to music, go kayaking or hiking, and I used to snowboard like a motherfucker as many days a year as I possibly could squeeze in. Also, lately, I’ve gone back to writing, mostly about music. (If you want to read about some really obscure bands, check out my blog; link in my signature below.) And lately I’ve gotten the brilliant idea to spend my money not on a house but on travel to exotic places like India and France.
Mostly, tho, I try and keep my schedule balanced so that I only work about 100 days a year. The rest of the time is mine, to do with as I will. And that’s really been my goal all along: to do what I want with my time, not what anyone else wants me to do with my time.
I’m 49. 3 years ago I could still outrun my kid, who is a D1 athlete, in the 50 and 100 yard dash. I had a knee replacement this year. Its cramped my style a bit, but I’m working my ass off to get all my strength and range of motion back. I know I’ll never be as fast or athletic as I once was, but I still have skills. I cant wait to get back to playing kid games.
In my 40s I still play video games – does that count?
Listen to the new, watch the new, read the new and talk the new…and the new sticks to you.
Skiing, hiking etc aren’t “young things”. In fact I’m sure a lot of young people (teenagers) would consider them to be “old fart things”. Especially hiking.
I don’t know why you’d imagine you have to give them up as early as your 40s.
I still watch the Simpsons, Futurama, Family Guy, The Daily Show, The Colbert Report, and exercise every day. I also make bad puns and strange jokes.
I’m 67
So ???
When I was in fourth grade, I told everyone I knew I wanted to spend my life studying ancient Egypt, Greece, and Rome. After I first read The Lord of the Rings, I knew I absolutely had to study ancient languages.
Now I’m 32, and that’s exactly what I do for a living. Though I’ve since added ancient India and classical Sanskrit to the mix, too.
I’ve always loved strategy games of any kind and play video games wherever possible. So I became a game theorist: I figure out the rules, payoffs, and strategies of political and economic interactions in the ancient world. It’s like watching momentary flashes of a Sid Meier’s Civ game and piecing together the rules that you can never directly observe.
Everything that I love to do today is pretty much directly connected to things I loved when I was a kid and would have never given up.
Except the hard liquor.
I hear ya. Once I hit middle school, I quit the sauce, too.
Skiing is for old folks, isn’t it? If you’re over whatever the appropriate age is and you try snowboarding, you are referred to as “grays on trays”.
I have at my advanced age a Facebook page, though I mostly ignore it and just send myself items of interest to save, and look over. I find great songs on youtube and send them to myself. My daughter told me her friends are flummoxed by some of my choices - “yr mom is into Marilyn Manson??? " (just for Halloween, kiddies) -” MY mom just has pictures of dogs and cats on her Facebook page." I suppose I should be interested in things that are more ‘age appropriate’, but I honestly do not know what those things should be.
But what percentage of any actions (preferences, hobbies, etc.) are strictly “age appropriate” instead of being “typical of a generation”?
For example, when I think of “old people” in their 70s-80s, maybe I’d think of going to bingo, wearing those matched Alfred Dunner-style outfits if they’re classy (and maybe things like Quacker Factory outfits if they’re not ;)), having poodle perms and watching game shows all day or playing shuffleboard in Florida.
However, think of what those “old people” thought of old people when they themselves were younger. “Old people” in the 30s-40s didn’t wear Quacker Factory and watch gameshows all day. They wore really old fashioned outfits (perhaps including their stays) with hair in buns (not perms!). They knit (or collected stamps, or whatever); they’d never go to BINGO, and so on.
So perhaps “old people things” are different to each generation, even though the things themselves aren’t what that generation liked when they were younger.
The other day, I raked up all the leaves in the back yard. I ended up with a big pile of them, and I was tempted to jump in it, like I did when I was a kid.
I still like roller coasters, old Warner Brothers cartoons, and the occasional fart joke; but I think the real answer to the OP’s question concerns losing the inhibitions that we gain as adults, and returning to childhood where it’s okay to pretend you’re something you’re not, to sing along with the radio, to make up games, or to just play however you please. If you can still dance like nobody’s watching, to use a common phrase, then it seems to me that you are reluctant to give up the things of youth.
In the end, I didn’t jump in the pile of leaves. Not because I cared how I looked; but rather because it was late in the day and I didn’t want to have to rake them all back up again in the dark. I’ll admit to being reluctant to giving up the things of youth, but on occasion, my practical adult side does take over.
I like to sit around and read comics, play video games and the like, so clearly my reluctance is in a different league to your own :o