Teenagers are on the cusp of adulthood, but still naive and impulsive. Sure, there are several exceptions to this rule, who are intelligent young people with a great head on their shoulders and know what they should be doing. However, as stated, they are the exception.

For analogy purposes, let's look at the lioness and her cubs. She brings the cubs with her on hunts, allows them to stalk the prey and sometimes, even allows them to initiate the kill. She expects them to exhibit adult lion behavior, while never letting them take the full responsibility yet. Why? Because they still don't know what they're doing. They only know what they've learned thus far. She still needs to teach them the finer points of being an adult lion, before they are ready to be treated like one.

Human teenagers, like the lion cubs, know what they are supposed to do up to a point, but still require more teaching, guidance and object learning, before they can be treated like adult humans.

I'm fairly certain that as you've grown, your mother stopped holding your hand when you crossed the street. She stopped cutting your food up for you. She started letting you dress and clean yourself. These are all steps toward adulthood, requiring you to act in an adult capacity, while still acknowledging that you are a child and still in need of teaching before you can act in full adult capacity.

The faster you learn and the more responsibility that you show your parents you can handle, the more they will treat you like an adult.