A very very short answer to this is, when they are able to comprehend the difference in gender. That is when you introduce certain 'dos and don'ts'.
The long answer can get a bit complex because it depends on many variables, so let's get started:
Environment: As a teacher trainer, I have reports from my teacher trainees indicating children as young as 7 years old playing 'rape' in the classroom. This of course got the teacher to sit down and have the 'talk' with them since their parents did not. It may be easy to blame the parents but they could have learnt it from an older siblings. Thus, while teaching them about sex education at this age may be too early, the example above does show that the children already has an understanding or misconception of certain things related to sex education. As such, parents and teachers should take action immediately. Moral of this variable is, watch out and be vigilant towards the signs.
Curriculum: In general, every education system has a 'set age' where they would give the students a short course on sex education. My country's is at the age of 12. While this may be a little late for certain kids, parents can start preparing them also when certain questions pop up in books or reading. For example, why do people get married? Where do babies come from? etc. Generally, you would want to slowly ease the child into certain ideas before you give them the full version of 'the talk'.
Other aspects to this include their friends and the media that they are consuming. However, the general idea to this is, every child is different and instead of figuring out what is the best age, watch out for the signs as mentioned in the environment point above. Also, consider gradually dropping ideas related to sex education such as marriage, babies, etc in easy to understand 'bites'. This is to build upon their understanding slowly and avoid confusion for the child.
Hope that helps!