Some young people seem to navigate this stage quite naturally, while others get stuck.
I think it's important to encourage independence gradually. In the younger teen years, that might mean sticking with sports, clubs, or hobbies—especially during the long summer holidays. Then, from around 14 or 15, it's helpful to encourage some kind of part-time work, even if it's only a couple of hours a few times a week. If nothing else is available, working for a family member is still valuable. The goal is to build confidence, competence, and a sense of responsibility. By the age of 16, I think most teenagers should have some kind of summer job if possible.
I know a few young people who have become "stuck." In every case, their parents are kind, caring people who have unintentionally enabled the situation. For example, they focused on arranging counselling for their child's anxiety instead of also helping them take practical steps towards independence. There's absolutely nothing wrong with counselling—in many cases it's a very important part of the solution—but it works best when it's paired with something proactive.
For example: "I'm happy to organise counselling if you'd like it, but you'll need to contribute towards paying for the sessions." That contribution might come from washing cars, walking dogs, mowing lawns, babysitting, or any other age-appropriate work if a regular job isn't feasible.
I also think parents can do a great deal by setting clear expectations. If you're not in full-time education, then you should be working or actively working towards employment. Parents can absolutely help their teenagers find opportunities, but they also need to know when to step back. For example: "My friend is looking for a student to work as a waitress this summer—you should give her a call." Notice the difference: you point them towards the opportunity, but you don't make the phone call for them.
Support, encourage, advise—if they'll listen—but don't do everything for them. That's how they develop the confidence and resilience they'll need as adults.