What age is best to talk to your children about sex and babies

One of my closet friends told me her 4 year old son asked her about sex the other night and she was worried about how to answer that question. I have has alot of experience with this type of situation, i have a strong belief that you shouldn't hide anything from your children. I believe that if your not honest with your kids the results of them making wrong decisions are far greater. So I simply told her to tell him the truth. Its never to early to explain sex to your child, although as they grow the conversation will become more elaberate, the teaching will always remain with them. I have an 8 year old daughter who has asked me this question when she was 6 years old, the situation was baffling, complex even but I simply sat her down and told her about sex keeping it child friendly of course, but again i feel that when they ask you its important to explain the question. I have Come encounter with far to many young children having children, the problem with this is they have no idea what to do with a child or how to properly care for a child. Most of which had parents who did not speak to them about the results of unprotected sex. Which lead not only to bring a life into the world, but the possibility of contracting a very serious sexually transmitted disease. So with that being said as a parent my job is to protect  my little one from everything including herself sometimes. Being a parent doesn't come with instructions or a How to hand book, each parenting style is different but the  most important thing is knowing and understanding your child and making sure they know and understand you.