I told my DS1 when he was in year 5. Because it was coming up to the time where you could see little girls in his class starting to develop. I have never hidden my period from my kids in that there has always been pads & tampons in the bathroom and they understood it was because when Mum doesn't have a baby in her belly she gets bleeding like other women.
DS1 knew to never, ever tease anyone about their period or the fact that they had pads or tampons and that if he ever saw blood on the back of a girls dress/skirt at school to quietly let a female teacher know.