I believe around 3rd grade would be good as most 3rd graders are around 8 years old and are beginning to be more cognizant of the opposite sex and sex acts.
If you wait until what some call middle school; which is generally when the child is going into teenhood; many of them these days are already sexually active and it may be too late for a sex-ed class.
When parents register a child for pre-K or Kindergarten, the parent(s) should be given a syllabus of what the child will be taught in that schools sex education class(es) so that the parent can better PRE-TEACH the child at home in accord with the parents beliefs.
I believe sex education should be mandatory and it should cover EVERYTHING sexual that THIS society considers to be legal; INCLUDING the fact that there are others that pratice, what some like to call, alternative lifesyles; i.e. homosexuality [...]

Source: FindArticles – Search Alert – home schooling