Any age, really. You can introduce the topic gradually through learning about biology. Pollination of plants, for example. Or bird mating rituals. At primary school, we had an egg incubator where we could watch the live growth of a chicken fetus. Make it clinical and normal rather than this forbidden mysterious thing.

High schoolers should definitely be taught about safe sex and disease prevention. Also, consent and how to deal with unwanted attention, or even what to do after rape, dealing with shame etc. Heck, talk about masturbation and how it effects the body and mind.

It all needs to be laid out on the table so, in the future, these kids grow up into well informed adults and we can forget about data harvesting for surveillance.