It doesn't mean you child is born as an adult and so you're raising an adult. It means you're guiding your child toward adulthood whcih is where we all end up.
To me the most important element of it is NOT to follow the natural desire to make the child the center of the universe and spoil them. Because teaching a child that no matter what they do, you'll never give a firm consequence, and that your entire schedule revolves around them at all times, is setting them up to be ill-prepared for lessons that start as early as kindergarten and evolve through adulthood.
They are NOT the center of the world. And there ARE very harsh consequences for wrong actions in the real world. Ones that can endanger your future (Stuebenville "kids"). If this is taught in appropriate measures from infancy to teens, the transition to adulthood is much easier than if a person babies their child until they're suddenly booted out into the real world as selfish people with no concept that their wrong actions have negative consequences.
Don't forget that in many ancient societies, and even some modern ones, boys left home for good to be trained as soldiers as early as age 7, and even now, many boys that age take on leadership roles (we live in a heavy Amish and Mennonite area and five-year-old boys WORK every day, often unsupervised with their mandatory list of tasks) so it's not unrealistic to think of children as approaching adulthood. Yes, they need to play and have a fun childhood, but parents are raising kids to be independent adults in the end.
I personally think this "kids aren't adults until 18 and then they should still be taken care of beyond that" mentality is not giving humans enough credit. In my experience, maturity and sense of self are pretty fully developed at around 15 if a parent has done their job, and the last three years are sort of academic filler and adulthood prep time until the child leaves to start their own lives.