I have two points to raise here. One relates to sexual maturity. I would agree that some children hit pubity very early, physically. However the presence of pubic hair does not mean that a child is mentally or emotionally ready to cope with sexual interaction, and while physical maturation is easy to measure, psychological maturity is much more complex. It reminds me of a re-occurant debate in the world of horse breeding where people use two year old male stud horses in their breeding program and justify that the horse is sexually mature because he has full testicles and can impregnate a mare. The young horses typically have no ability to court a female animal who has to be sedated and tied down to let the clumsy youngster mount her, and the young male usually ends up with aggression issues and psychological instability later in life.
I'm not saying that humans are the same, but I think people should recognise that there is a difference between apparent maturity and actual maturity. Especially seeing as humans live in a much more complex social world, it would be interesting to follow the lives of young teenagers who are sexually active at a young age to see how they fair later in life.
My other point is that everyone thinks they are mature and responsible at 16, and parents always disagree. I am now 22 and I look back at my teenage years and think what a fool I was. My parents guided me against doing certain things, and at the time I was very angry with them... but when I look back now I realise how wrong my opinions were and that I didn't miss anything, except teenage pregnancy, which happened to all but about 10 girls in my year group at high school. I still feel that I have a lot of maturing left to do, and you don't ever stop, but I think it's easier for someone of 18 and over to step back from a situation and think about the best decision to make than it is in the younger teenage years, regardless of who you are. The process of maturing mentally is a gradual one though, and at some point teenagers are going to need to step out on their own regardless of what is best, and definitely by the age of 18 or 22 you need to live your own life.