How early should sex education start?

It depends on the sex education curriculum. I see sex education going beyond abstinence-only and safer-sex to encompass the following:

Self-esteem ... to value oneself.

Parents can help affirm their young children's sexuality, strengths and uniqueness so that they would grow up knowing that their significance is not dependent on acceptance by others and they need not conform to group pressure to feel worthy.

For the boys, it means that they don't have to prove themselves to be manly by having sex with girls and that it's okay to be virgins. For the girls, it means that they don't have to give in to sexual advances to feel loved.
Self-control ... to be able to restrain sexual impulses rather than yielding to them.

We need to recognise that feelings and impulses are not wrong. But we should not allow them to dictate our behaviour.

Set your own criteria on what you should and should not do (based on principles and values). It is important to establish principles beforehand as to what to do when caught in such situations because emotions may dominate your being and rational thoughts go out the window!
Respect for Others ... their values, beliefs and feelings.

As a good friend, you should not to take advantage of or impose on others. This is especially important for boys to learn because it is transferable to making sexual advances.
Responsible Behaviour ... to know that our actions have consequences.

Do not automatically bail out a child when he or she encounters problems. Allow a child to face the consequences of his actions will help the child to realize that his actions have consequences and to think before he acts.

Sex is powerful. It may restrict the life choices of you, your partner and the child to be born. You may contract AIDS ... and possibly lose your life! You may be pregnant and have to stop school to deliver the child. Who is going to support the child financially? If you and your partner have to stop school so as to work, you will have less job opportunities and less income in turn may mean less opportunities for the child.
Assertive Communication ... how to say "No" to a guy who wants to have sex.

It is a sad to read the 27 February Straits Times article on "I can't make them use condoms". We have to learn to think & make wise decisions, learn not to be intimidated, and not to give unthinking and unquestioning compliance. We can do all these without feeling guilty or hurting someone's feelings.
Conflict Resolution ... to learn to work out differences especially with family members.

Conflict resolution is important for both parents and children. For parents, to communicate that no matter what they are committed to their children ... to provide a model ... that relationship calls for commitment. For teens to learn to express their opinions and desires in ways that show respect for their parents and not to run away into possible bad company. In September 1999, a 14-year-old girl, who ran away from home, was allegedly raped and forced to perform oral sex on a Thai construction worker at Bedok Reservoir Park.
Evaluating Media Messages

Media messages communicate values. We need to look at and listen to them critically and evaluate the values being communicated. We must not adopt the lifestyle presented without thinking.

With the above curriculum in mind, sex education could begin as early as kindergarten. If we, parents do a good job in our children's early years, and then we would have more confidence that they would not engage in pre-marital sex.