I'm not the original person, but I was interested and did some digging myself, so here's what I found. I'm primarily citing this paper which seemed to cite a lot of other papers to back up its claims, compared to many others, that just utilized a single survey's results

The paper specifically mentions education all the way down to the preschool level, whereas many other studies didn't do anything below middle-high school.

Parents, teachers, families, neighbors and the media all have important roles in the sexual education of children and give children sexual education from birth without even noticing that they are doing so. Studies have confirmed that sexual education is a lifelong process that starts at birth.

This is the key point: Sexual education is already effectively taught in many ways in non-educational settings, often with traditional heterosexual norms instilled. (e.g. general discussion of relationships and attraction, consent, mentions of people "trying to have a baby," things like that) This is education that the respondents themselves did not consider to exist (the majority said they believed sex education of any form did not begin early in adolescence)

However, most of the general resources I can find around how official sex education curriculum are developed, how parents bring up these topics to their kids, and what kids are actually comfortable with discussing themselves, seems to point to an age-appropriate level of education, based on what they're likely to encounter at their given age range. (e.g. a very young child may be taught to say no if someone asks to see their privates, whereas a young adult may then be taught how to properly use various forms of contraceptives, with the context of different sex positions, because that's the age within which they're most likely to engage in those different positions.)

It seems like the age-adjusted measures work best not because they necessarily bring harm if taught to younger individuals (although there's significantly lacking data on this specific age range and being taught a more comprehensive sex ed curriculum) but rather that it's more possible to teach it to students as they get older, because they form a larger body of existing knowledge around the topic from peers/media/family, that provides the context required to be more easily taught, and they become more comfortable discussing such topics as they grow older and have a larger existing understanding of them.

You could try teaching an extremely comprehensive sex ed curriculum to students who are much younger, but they would probably just be too uncomfortable to actually care/pay attention/truly learn, is what the evidence I can find seems to point to.