Never too early. Dd was being kissed regularly for a short period of time in 4th grade, against her will. She finally got upset one day and told me she did not want to go to her bff's house because he had been kissing her and she didn't like it. I am not sure how much she spoke up for herself, or if she was just shocked and uncomfortable, so the moms discussed and policed it and the friendship quickly cooled that year.
CM, it really disturbed me as well, that the kid did not get the "no means no" thing, and repeatedly would attempt to isolate dd, even when the mom was thinking they were under her nose. Hide and seek in the home, 10 seconds behind a closed door and he was on her! I was so upset about the big picture of that and how he was really crossing a line in so many ways and his parents had their head in the sand, I forgot all about the allergy risks! He was her bff and never ate PB, really, if she was a round, though.
Anyway, never too young. he had other ideas as well. No kidding. Thank goodness dd hated it and told me! I shudder to think if she had liked it. They had been in "puppy love" for years before that. She says he is a bit of a jerk these days. No kidding?
So, this was 4th grade. If he had happened upon a girl equally interested, more would have happened.
Overall, I have for a long time discussed the risks of kissing and the FAs. Our schools cover a lot of sex ed in the 5th and 6th grade movies. But I need to re-cover it with dd. She was not emotionally ready and was grossed out and tuned out. Failed the test. So, she will need a review. Right now she cannot talk about it. Is not interested. She presently has a firm resolve to adopt children because she is so repulsed by the deed.