I would bet that 100 years ago your average 13 year old was more mature and "grown up" than today's 18 year old. Society has a special knack at extending childhood.
High School graduation is touted as some grand accomplishment. All you have to do is show up and turn something in and you will get your diploma. How many kids think this is now "difficult."
You can't work (for all practical purposes) until you are 18 because of the child labor laws. Try finding an 18 year old employee that is worth a damn. They are few and far between.
Now 16 year olds are not mature enough to handle driving a car.
We raise kids to be irresponsible and worthless and are surprised when that's what they become. Instead of making them responsible we simply believe they will learn these values by aging.
The problem is not the age. It is the lack of societal expectations and the enforcement of effective consequences.