I know from very early age the basics, like 7? My parents had a book written by some experts, which gave some information for kids between 6 and 10 I think. They also had a book for kids between 11 and 14, which then gave more information.
The first book explained a bit about both man and woman bodies and pregnancy, including bit of info about period, but not like actual sex. That was in the later book and my parents discussed sex more with me round when I was maybe 11 to 12.
Plus I had really good sex education. Which included session about just how female body works when we were about 11 to 12, which explained the circle, ovulation, period etc, and we were given information on various hygenic products and also a calender to mark the time when we actually get the menstruation and to keep track. I think this talk about this age is appropriate, as some girls can get menstruation when about 12.
We also had a talk about sex and STDs, including how to protect ourselves from STDs, which they are, what they look like, when they appear -they used photos, how they can be treated. And we had session with HIV positive person too.
We had a talk about how to prevent pregnancy too, with either doctor or nurse. Explaining various methods, what can be the problems with each one. And we also had a talk about abortion. What methods are used for what stage of pregnancy etc.
But I am not from the UK, so this is different for me. But I think it was really good education though provided by school, and some of it would be pretty hard for my parents to provide, e.g. the detailed information about STDs with photos. Back then, most people did not have computer and internet, yet