Prospects in life is the answer.
The article goes over how independence in adolescence could help reduce people's anxieties, and closes with the following which sums up the article rather nicely
> “When we are controlling a young adult’s experiences, and they go without that full range of emotional experience, we’re actually curbing people’s opportunities to live full lives, and have the full range of human experience.”
Observe then, that absence of prospects means lack of independence. There is no reasonable form of independence when you need to work 9-5 just to cover your most basic living experiences.
> “It’s about being proactive and not feeling like you’re a victim, how you can control some things, but you can’t control everything,”
But as a person, you are living in our society, and you are bound by the rules and the situation you are born into, not everyone has the opportunity to explore and put themselves in relatively difficult situations without significant consequences because that is often very taxing, especially for the destitute. It is not possible to circumvent one's economic background, or remove it from the equation. Getting your clothes dirty, putting yourself in harm's way (even minor) can be very expensive, and thus taxing.
It is absurd to expect people to go out of their comfort zone when doing so has so many tangible negatives for them.
I come from a very poor family, and all of my young adulthood was spent clawing out of poverty through education. Sure, I have overcome the anxiety described in the article, but it wasn't because I had independence.
I didn't have a car to go to university, my family left me there in the morning, I didn't have a job for a non trivial portion of my studies, I just didn't eat until it was time to eat at home and only if we had something. I didn't go to trips because I spent my summers working in internships trying to escape poverty. I didn't explore the area I did my master's in because even taking the metro meant that I'd have to choose between a meal and a trip.
I am by all means fearless and bold because I have marketable skills and a hunger for better. By all means, I was lucky to be born in a time when my skills are useful to the system.