When should sex education begin?
At birth.
From infancy on, children should be allowed and encouraged to accept and love and respect their own space and their own bodies. They should be taught to respect others' space and bodies. They should be allowed privacy, but not so they can HIDE things - if they are encouraged by positive responses to be very honest and open, they will be, sometimes TOO much so (but more is better than less).
Their curiosity should be encouraged, but balanced with respect for others, and the concept that certain things about themselves, others, and their bodies are precious and private.
I believe they should be taught that sexuality is best in the context of deep and committed love between two people.
I believe that this lays the groundwork for open dialogue about sexuality when their open and honest questions are asked. I believe we should give them direct answers, and in the younger ages - under 10 - unless there is some unusual circumstance, those answers should be short, to the point, and not overloaded with scientific detail.
I believe they should be encouraged to participate in sex ed classes at school. However, as a parent, I would like to see the curriculum first. If I didn't agree with the curriculum, I would still allow them to attend the class, but I would clarify where I disagreed with the class, and why.
I believe sex education in school should begin in about the fifth grade.
Just my personal opinions as the mother of five grown kids who seem to have a healthy perspective on sex and love and relationships.