You know, way back in the day people use to get into serious marriage type relationships at that age so it isn't unnatural for the youth today to do the same. Some people mature faster than others.
I was going to comment something similar to this. It is true that biologically speaking, it is natural for teenagers to start desiring/being drawn into relationships.
I think the major difference between youth of today vs youth of the bygone era, is say, 100-150 years ago, kids from about the time they could walk were trained to work/help out around the house just for the family to survive. Girls learned to cook, sew, maintain farm animals, laundry, keep a house, repairs, etc. Boys learned their father's trade or were apprenticed out - out in the fields farming, black smith, mills, etc. If they were lucky, there was a school in the area they could walk to to get education to a point. So by the time they were teenagers, both boys and girls had a working knowledge on how to keep a house and provide for a family, and were eligible to be married off.
Today, most kids are in school until they are 18. What they know about taking care of them selves depends on what their parents may or may not have taught them - and today's parents don't always teach them enough of the basics of how to survive on their own in today's world (not just cooking and cleaning, but also need financial management, job seeking skills, etc). Rather than learning, kids watch TV, play on the computer or their hand held games, "social network", etc. This lends to a lesser maturity level by the time they graduate compared to youth of the 1800's. There are parents who do their best, and some kids that naturally tend to want to "grow up" sooner, but I would say on average, youth of today don't mature until much later in life...at least in this country. The rules of the 1800's can be found in other more primitive cultures around the world where families still need to work day-by-day to survive.