I've spent a considerable amount of time thinking about this, ever since the topic came up on reddit about why so few people in their 20's seem to "have their shit together."
I think it comes down to two reasons. The first one is huge, but its essentially luck. I think the reason I'm even afforded the opportunity to take my 20's seriously is almost completely happenstantial.
Looking at my friends and (distant) family I sometimes feel guilty because nothing bad has ever really happened to me in my life, whereas they've had to put up with all kinds of weird shit. For many people I know not being able to take their 20's seriously as powerhouse career years is sort-of excusable. They were spending them trying to survive, or raise a family all-too-soon.
But me? I'm young (24), well educated, I live in a huge Victorian-era house near main street nearly for free, programming job, walk to work every day, have a book deal (boring HTML5 book), brag, brag, etc, etc
But somehow I feel I'm just infinitely lucky. Lucky that my parents are two normal, well adjusted people. Lucky that I could be awkward and nerdy as all shit through middle school and high school and nobody was ever unpleasant to me. I was never bullied. No weird drama ever entered my life. I exited college with $0 but debt-free, thanks to Bank of Dad, who carefully engineered my experience to be basically broke but never in-the-hole so long as I worked (got an internship every summer and winter).
Lucky lucky lucky. Thanks everyone in my life so far. Really.
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On the flip side I think that such a lifestyle (very career focused 20's) is seen as unattractive to a lot of 20-somethings. At the least, I think most people in my age range would describe people like me as boring (though I am never really bored). I spend most of my days reading, writing, sitting in cafes, programming, or doing art things. Right now and for several more months, its nearly 100% career activities. If I had to put together a dating website profile, it probably wouldn't look particularly attractive to other 20-somethings.
Put another way, most of my hobbies are either career related or solitary acts. Not that people can't appreciate them, just that none of them are exciting, and most of my 20-something friends (none of which IRL are programmers, I lead a lonely career here in NH) that proclaim me to be one of the few who has their shit together would also not want to be me.
None of them want to work desk jobs and then go home and work some more. They want to work in food/bar services or have a whatever-job, drop it at 5pm and go do fun 20-something things. Career development it seems isn't fun to most 20-somethings, socializing at bars is, moving out to Montana to climb things and work for a harvest season is, saving up to hide away in Peru for a year is, writing music with friends while working retail is, but not career development. Not to disparage these things, it just a sampling of their goals.