The school should have let you know about the talk, but I do think it is important that kids know about this stuff. I work in a school, and I can tell you for sure that we do have 9 year olds having their periods.
I can also tell you what talk I overhear on the playground - what boy likes what girl, who is ‘going with’ another (these ‘steadies’ last for a few days usually, and consists of them eating lunch together). Some kids definitely know what is going on!
This is a Catholic School, and these are 4th, 5th, and 6th graders. (9-12 years old).
I must say my kids all knew what was going on pretty early - 3rd grade probably. When they asked questions, I would give information. A little at first, and then more as the questions came. They wanted to know, and they were going to find out, either from me or from their pals. Who better to teach them than me (as their pals probably had mixed information, or just made it up!)
My 14 year old has asked a girl out to a school dance. My now 13 year old had girls calling him since about age 11. My now 11 year old is very well liked by most of the girls, and several have called to have him over to their houses and work with them on school projects, etc. I can see where this is going. We have talked! In my experience, it is the girls who are quite advanced and very pushy!