And if parents wont teach it?.
My sex ed in the UK started at 10, with the very basics. At secondary school (12-16/17/18), it became more specific.
IMO that’s a sensible age and progression.
It is the responsibility of the parents. It is not the same as teaching math because educating a child in matters of sex also has a component of morality to it. The parents should not just tell him how things work physically; the child also needs to learn about respect and personal responsibility.