Sex Education does not only tackle sex. But the whole reproductive system and it's functions. For a mother of two opposite gender. As early as 3 years old I teach them why is the boy has this and why on girl is the other way around. At this age you can only teach them their private parts and why they are called private. By then, your child will not will be curious about it as they grow older. Sex Education should comes first at home. Second is in school where they will discuss the function and more scientific terms in every parts of it including sex & pregnancy.
I was also introduced to sex, particularly the reproductive system when I was in grade 5-6. At that time, we were old enough to somehow grasp what was being discussed, but of course there were still a lot of questions in mind. It should be a step by step introduction, depending on the age and ability of the child to understand each specific topic about sex.
Sex ed should begin in grade school. Not sure what the right age is. But based on experience, we were taught as early as grade 5 during Health class. My mom also gave me a talk about the birds and the bees when I was in grade 6.
There are a lot of ways and medium to discuss it nowadays. Feel free to be resourceful.