I recall getting taught sex lessons during the first year of secondary school and thinking how stupid it was then since we didn't get told anything we didn't already know. Embarassing also. (I also recall being told we all HAD to ask a question so I thought of one and didn't get a very satisfactory answer. I don't think the teacher knew as much biology as they thought. I recall thinking why did they insist on a question if they couldn't answer them anyway.)
So based on that experience from decades ago I'd suspect if it is to be of any use it should, latest, be the last year of primary school, which would be 11. 10 might be ok. 9 is possibly slightly young but if you wish folk to treat the subject as a normal part of life and not something to be embarrassed about, I can see why it might be considered. But yes, maybe a little young to be force fed the information rather than have questions honestly answered.