You are using a biological definition of adult to determine adulthood for our society. And just a couple of generations ago, it was perfectly ok for men to beat their wives, women were bsically property, and half of the children born didn't live to see adulthood.
And please point out where I said anything about waiting until you are 40 to have kids. For some reason you think its either having kids as soon as you hit puberty or wait until you are 40. Most people have their children in their late 20s or early 30s. The average life expectancy has gone up. In 1900 in the USA, life expectancy for men was 48.3 years and for women it was 51.1 years. In 1990 the life expectancy for men was 72.1 and for women it was 79.0. So we have added more than 2 decades to our lives, on average. So the fact that people married and had kids at such an early age is not relevant. They had to because their lives were much shorter. Also, since we were an agrarian society, children were seen as field hands that didn't have to be paid. Farming is now only a small portion of the population's livelihood and is largely mechanized. So the extra hands are not needed.
In order to do well in our society, one must be educated. These girls have chosen motherhood over education, thereby lowering their chances at a successful life.
In the book Future Shock, there is a chapter about the changes in our world. If you create a timeline based on events and inventions that changed the world or society instead of on years, the middle of the timeline would be around 1900. In other words, the average life for people around 1900 was more similar to that of ancient Rome than to modern times.
Using the standards of "a couple of generations ago" doesn't hold water when discussing what happens today.