Just because we survived as a species does not mean that young girls lead idyllic lives when they were married off or sold into slavery at 13. When whole populations could be wiped out by disease, famine, or war, producing lots of babies was important to survival. Even then, there were pretty strict mores regarding sex. Violating them could mean a death sentence or being ostracized.
Typically, as societies become more successful and wealthy, women have fewer kids and wait until later in life to have them. And even though we are physically able to produce children at young ages, it doesn't mean it's a good idea. Our brains aren't fully developed until around age 25 or so. Because of this many adolescents are practically wired to make crappy decisions. Sometimes the natural consequences are what's necessary to keep them from repeating the same mistakes. Other times some intervention or prevention is necessary. Lots of teens are definitely at risk for suicide. You don't always want to wait for them to work it out themselves.
I have a 15 year old son, and a 12 year old daughter so I'm right in the middle of this. Even though my son's hormones are raging and my daughter's are headed in that direction, they aren't even close to being physically or emotionally mature. They do not have the means to raise a child on their own. Of course, there is such a thing as birth control, but the chemical methods have bad side effects and potential health risks when used for a long time. Non-chemical methods tend not to be that reliable. So delaying full blown sex until their later teens or early twenties is something we'd like to see. Definitely past 14. Past 17 is probably pushing it, but we can hope. 17 is the current average for when kids become sexually active.