I'm not a child psychologist; I'm not any doctor or neuroscientist. This is purely from my point of view and not meant to be any scientific or authoritative viewpoint. This will ramble a lot, so I apologize for that in advance.
Children aren't as innocent as you think.
This idea of “innocence means you don't have knowledge of sex” is just a strange form of sexualizing children.
That's also not okay. Innocence isn't ignorance; it can be but isn't inherently. Children may not know all the terms or have the concrete awareness like adults do, but to think that they're completely unaware is just ignorant. Most likely, you weren't innocent as a child either. I remember being a kid too. Many of my friends and I were curious about bodies from a young age, both each other and just people in general.
Don't act like children aren't curious about that; the trope of “playing doctor” exists for a reason. Children are naturally curious and creative, which likely will involve naked bodies. It may not seem as sexual to adults and/or telios, but it is in a way.
Not every kid has an opportunity to talk about sex or personal issues in a super clinical and SFW way; that's a valid way for them to find out.
If you only talk about sex in a super SFW school setting, that's not going to be enough for many kids, which will leave them unprepared for actual sexual situations, which, surprise surprise surprise, are not like the scenarios covered in sex ed. You're going to have kids, especially pre-teens into early teens, who are not exactly innocent with their sexual desires; yes, they have sexual desires. They're going to want to know about stuff like blowjobs.
There's a whole other discussion on how heteronormative sex education is, which alienates gay, bisexual, or lesbian children, but that's a bit too much right now.
Additionally, there's not that much difference (mentally) between 16 and 18 years old, if we're honest. There's a whole other discussion about developmental ages and the developmental differences between a 16-year-old and an 18-year-old, but this isn't about that.
If you claim that children are innocent and being in any way sexual with them at all is bad, you're just fetishizing innocence.
My firsthand experience
Note: Please note that the names have been changed for information security reasons. Additionally please do not ask about the real identities of the mentioned children.
I used to be a camp counselor at a camp where it was mostly young boys — no, not boy scouts — it was mostly segregated by gender, boys being on one end of the camp and girls being on the other. For this reason, I'm mostly speaking about my experience with the boys. Most of the boys were from around 13 to 15, with 1 being about 12 years old. It was an enjoyable time for most of the boys; many were bullied or didn't have the best experience in school, so I tried my best to make their experience fun and make sure they had a good summer. I was the counselor for their cabin, meaning I slept and showered with them and all that.
Obviously, being older and more developed, the boys were somewhat curious about my body. For some of them, they had only seen another naked man in porn, not what most bodies actually looked like. They were curious about what being “grown up” is like, and for the most part, I was happy to answer their questions. For the questions that were a bit too inappropriate, I answered, “You'll know when you're older.”
“Hey bro, am I going to look like you when I'm older?”
“Am I going to have a dick like yours when I'm older?”
“Hey big bro! Look!”
I was called “bro” or “big bro” a lot, and they generally saw me as an older brother as many of the campers had come back several years in a row — as did the counselors, myself included. Boys are naturally curious, and if you try telling them “you shouldn't be learning about that,” all they're going to do is look in places that maybe aren't the best for them, namely porn.
Boys, especially teen boys going through stages of puberty, are at varying levels of insecurity. Some are the cocky dominant types who are more than happy to show off their bodies to anyone who will see — whether willing or not — and then some are shy and insecure about their growing bodies. They asked me about what sex feels like, what a blowjob feels like. Some bragged about how they already had sex or a blowjob, which caused a lot of admiration and awe among the other boys. Whether that was actually true or not, I couldn't tell and really didn't care if it was or wasn't.
I've had to talk to many campers about their bodies and about how they shouldn't worry so much about being “enough” because everyone is different, and that's okay. Something repeatedly came up with the size of their penis, a subject that makes even adult men uncomfortable. I frequently explained that they're in different stages of development and have different family histories and that sort of thing.
There was much rowdiness and general roughhousing that boys do, and there was teasing about “boners” and other stuff. I had a talk with the boys about where that's appropriate (in private) and where it's really not (anywhere in public.)
In particular, I had one camper Kyle, who was very much a young exhibitionist. He was confident, cocky, and very aware of how he looked. He was only 14, but he was decently muscular for his age and starting to show some hair. He was incredibly horny all the time, as most teenage boys were. The thing is, he wasn't quiet about it; he knew how inappropriate it was; he didn't care. He was certainly the “dominant” type who would later grow up to be an “alpha” archetype probably.
One of the other campers, named Hunter, was very curious about pretty much everything Kyle did, including the stuff which got Kyle in trouble. Hunter was a bit shyer and reserved; however, he was obviously interested in Kyle. Whether this was homosexuality or not is something I'll leave up to you.
I walked back from the counselor's meeting, assuming that everyone would be at an activity. Instead, I found Hunter and Kyle comparing each other's dicks. You know the “mine's bigger” sort of thing.
I turned around and walked away, admit it or not; it's something boys are curious about. The fact that Kyle was a bit more active and upfront about it wasn't my problem. I knew for a fact that while Hunter was shy and Kyle was pretty outgoing and dominant that neither of them knew to cross boundaries. You'd think that Kyle would be the one to be a bit of a bully, and he was a tease, but when he was told to “stop,” he immediately did.
Sorry for rambling; the point here is that if you're assuming children are innocent, you're just doing a disservice to children, as well as their development at large.