Premise: it takes people longer to develop into autonomous adults capable of independently navigating the world than it used to.
If we accept that as true (I'm not entirely convinced, because what it means to live independently is so radically different over time it's hard to really compare, but generally accept it as a roughly true premise), what might be the causes:
Possible cause: mass schooling is only about 100 years old. Only in the last 60 or so years have we moved to nearly everyone attending, and only in the last 30-40 to nearly everyone attending for the full K-12 experience. We have many times more people starting even earlier, continuing much later with college, attending after school programming, and summer programming. The school day and year have both gotten longer. Schools have become more standardized across the country, and parents are more involved in continuing school activities like homework after hours than ever before. Each generation is more schooled than the one prior, and each is the most schooled generation in history.
Possible cause: technology advances at an exponential rate, making what's valued in the market change rapidly. Schools, by their structure, cannot ever advance as fast, so are always falling farther behind and creating a larger gap between what schools incentivize and what the market incentivizes. While schools still treat "screens" and the internet as playthings good for entertainment and naughty distraction and maybe a few stale lecture videos, the market treats them as the core of everything. Schools don't understand software, the world doesn't understand anything else.
These two causes combined mean kids (16 to as late as 30) emerge fully engaged in the digital world only as a fun social life sort of plaything, totally digitally competent but unable to translate that competency into value since their paradigm for nearly two decades is fixed where software is seen is fun stuff and books and facts and rules seen as valuable stuff. Kids are being taught less relevant things than ever longer than ever more separated from the world than ever, therefore it takes them longer than ever to "grow up", because they need to overcome all that, and then begin the process that should have started at adolescence of finding out what they value and how to do it.
If the above is even partially true, the good news is tech is eating school too. You can't shut it down, and more and more kids are tuning out of school and tuning in to software, networks, etc. in a way that can't be contained by barbed wire and truancy officers forever.