IMO Having sex before your brain has fully developed its ability to think logically is just silly. Of course not many people would agree with me that people should wait until they're 25, but I've noticed that around this age people start thinking logically about their futures.
Also some research seems to suggest that teens (especially girls) who engage ins sex are more prone to depression and suicide. I know I felt like committing suicide after several relationships fell apart almost immediately after sex. And that was between 18-21.
I also read statistics which suggest the average high school relationship lasts about 3 months. That means you have 16 opportunities to bond sexually and have your heart ripped out and crushed by teens who are only beginning to learn respectful behavior.
In my county alone a recent study by WHO stated that 25% of people between the age of 15-25 had an STD, but they estimated it was more likely to be around 75%. So you tell me who is more likely to get tested or insist on a condom? A 15 year old girl or a 25 year old girl?
Finally, I think in an already narcissistic society we should be doing things to empower our youth in non-sexual ways. Our teens struggle with finding themselves and bad self image enough without throwing the inevitable rejection after sex into the mix.