Children are exposed to sexual information at a young age, regardless of how the school might try to shield them. It is important for the schools to provide accurate information early, instead of allowing false information to proliferate. Sex education should be treated as a health and safety issue, not a moral one.
Young age is an extremely subjective standard. 1st graders do not need to know the mechanics of sex or how to use a condom, but they could be taught about how to report sexual abuse to an adult.
I believe that sex ed should start young, and should start with basics such as anatomy and consent. Young children should be told what is and is not appropriate, and what parts of their body are considered "private." As they get older, they can be taught more details about things. Graphic information should wait until at least middle school, but knowing what a penis or vagina is, is not inappropriate for elementary school.
The term "young age" is rather vague, but as long as the subject matter is tailored to the age of the children, it is fine. Many parents do not educate their children or young teens about sex issues at all, leaving them more likely to be taken advantage of, or end up in a situation they do not understand.
Sex Ed in the form of learning about your body, how babies are made, and the diseases that are out there are very important for American culture. It is why we have children later than other countries and why we have a better knowledge of STDs. However, I do believe the moral and personal side of Sex Ed (specifically the act of sex) should be with the parents or guardians. Parents should be in charge of informing their child about the biology and the schools shouldn't be responsible to inform about the morality.
Children are not ready to learn about sex education in early primary school years. Until children get closer to puberty the lessons would have little meaning. We have found that the best way to have a student retain information taught is for them to be interested in it. By the preteen years kids are showing an interest but not at 5 or 6.