I would say it’s a process over the course of their teenage years, not so much a game changing conversation one day requiring them to adjust rapidly.
Do this by preparing them mentally, emotionally and physically i.e. slowing adding responsibilities and expectations of an adult to their routine and impressing upon them the necessity and benefits of independence at a certain age.
When the time comes they thoughtfully and purposefully step out into the world prepared to face the trials of a young adult. Always understanding you are there for them should they need you, but feeling empoweed to face the hurtles by looking inward to their own resourcefulness.