In a way, I see it more as a problem of having the right relationship than being the right age. Family planning is use of birth control, but it does also encompass infertility concerns to some extent. Many clinics provide a certain amount of infertility care to those who want to get pregnant and are having difficulty. Family planning includes periodic abstinence and other methods of preventing pregnancy, as well as your standard contraceptives. Of course it's possible to abstain from sex.
Many people do.
They do their whole lives. Not so much in our society, but in other societies, it's been very common. The most important link is that teens who have a baby very often drop out of school and shorten their education. As far as whether more educated people have more or less sex than less educated ones, there probably isn't a lot of difference. The methods don't cause cancer.