Laws and Adolescence

I have noticed that laws regarding adolescence are not fair at all. It seems like adults are always favored to teenagers. I don't understand why adolescence have to be treated like second class citizens. What is so special about eighteen? Why do we have to be eighteen in order to be treated as an equal. For example, if an adolescence is slapped by an adult, it is not considered a crime unless it leaves a mark. However, if that exact situation happened to an adult, the culprit would be charged with assault. I just can't comprehend the logic behind this. I know a number of adolescence who are more responsible and generally more aware of the path of their lives than some adults. There is a false stereotype that somehow directly correlates age and wisdom, and I believe that this is an unfair assumption that seems prevalent when it comes to laws concerning adolescence.