Why are people growing up slower these days?

Back the day people were married with kids by the time they were 16 or 17. Now young people are expected to go to high school for 4 years and go to college for 4 years. People who don't at least finish high school are considered drop outs and usually have a hard time financially. Back in the day, people only needed to go to school until 14 or 15 and then they got a job and got married and started a family. And now if a 16 or 17 year old dates a person that is older than them, such as age 30, the person is considered a pervert and the 16 or 17 year old is considered an innocent child. What changed this? Why are 16 and 17 years considered children now days? Why did high school and college become necessary to go to if you want a decent job?