Not sure.
I guess men are biologically programmed to find women the most attractive when they are most fertile and that would be late teen years. Maybe it's something about the 'innocence' or the youthful energy as you suggest. Maybe it's a combination of all of the above.
One thing I will say is that most men under 40 who are saying they would never fuck an 18 year old are liars. If a hot girl of that age was coming on to them there's no way most of these guys would turn them down. Most men will fuck almost anything given the right situation. A lot of virtue signalling itt, especially from 20 year old boys saying they think 18 year olds girls look like kids ffs

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You can legally go two years younger in the country I live in so I wouldn't think twice about an 18yo lol
I think peak fertility in women is in their early 20s, though I suppose it's a difficult thing to collect data on so who knows.
I moved recently and have been going to a new gym that happens to be attended by alot of young people (just due to the demographics of this area), like high schoolers and etc. I look younger than I am, appearing to be perhaps in my late 20s, am fairly good looking and have a girlfriend who is in her late 20s but looks like she's in her early 20s. The sum of these factors, combined with the fact that it's always the same crowd at this gym, has made some of the girls feel comfortable enough to try to come up to me and engage in conversation, some even asking if I have snapchat or instagram (never when my girlfriend is around). I'm nice to them but also not very talkative and have never given them any of contact info.
It's interesting, despite some of them having nice shapes objectively speaking, there is something unattractive about all of them, even outside the notions of right and wrong, legal and illegal, etc. They just seem sort of awkward, and stupid, even the way they stand or walk or the expressions on their face. Just something unsexy about them. And the ones that have tried to talk me were just awkward and annoying.
Whats fucked up is that I see alot of guys (adult men), especially black guys, trying to chat up some of the girls that are clearly under 18. And I see alot of guys staring at them while they are exercising, etc. There's even this one middle eastern dude that wears dark sunglasses while working out, and I'm almost positive he's doing it so he can stare at them unnoticed (which is what he appears to be doing).
Far more attractive are women in their 20s, they just carry themselves in a far more sexually attractive way.