I'm not trans, so I can't speak for trans people (not that any individual person could speak for an entire demographic group). And to pre-empt the follow-up question "then what's with the pronouns": gender is multi-faceted and complex, pronouns are just one aspect of it. Think of it as gender non-conformance. Like wearing a dress as a bloke.

If I had to hazard a guess for what might be causing the correlation you're seeing, I'd assume that being trans usually comes downstream from reflecting on social phenomena and cultural expectations. Being trans is by definition not the cultural norm (even the word itself implies some form of "misalignment" of identities and cultural expectations) so if you are familiar enough with it to claim it as your own identity, you probably did a lot of research into it, especially if you're from a generation where it was more of a taboo subject and not acknowledged in the broader frame of cultural references (e.g. good luck if your only exposure to the concept is from films like Ace Ventura). This can lead you down a rabbit hole if you try to understand not just that one aspect of your identity and experience.

Your actual question comes off as a bit upset (but again, that may be cultural - I'm not American nor is English my native language although I did pick it up at an early age) so let me rephrase it in a way that makes me more inclined to answer it: "Why do you think that is?"

This still feels somewhat like proving the null hypothesis as "it's in our genes" is not normally the go-to explanation we accept when wondering about any random part of human behavior but let's start by turning it around. Sure, we can make up all kinds of just-so evopsych rationalizations why human males should be sexually attracted to post-pubescent young and healthy human females but the same reasoning would also predict a preference for a prolific pelvis (making it more likely they successfully give birth) and pregnant women (demonstrating actual fertility) or mothers (demonstrating child-rearing abilities as well as fertility) and so on. Ultimately these are all just-so stories to rationalize a pre-existing assumption about human behavior that contradicts actual archeological research (which adherents often explain by claiming archeology has been corrupted by ideology but let's not get into fallacious claims of being "free of ideology" and where all of that ultimately leads).

The answer then is simple: I say fetishization of youth in women is cultural rather than innate because it is not a consistent phenomenon throughout history nor even globally in the modern age.

It's important to distinguish between the two factors at play in child sexual abuse: sexual attraction (i.e. pedophilia) and power dynamics. This isn't unique to child sexual abuse. Regular rape also often is more about power than attraction. Everyone is familiar with the concept of prison rape and historically, sometimes even today, a male rapist of other men in a prison is not by default considered gay or effeminate and the act may be seen as demonstrating dominance, demeaning and emasculating the victim.

The reason I'm talking about "fetishization" is because our culture (and US culture particularly so) first of all very much embraces narratives of dominance as a positive, from competition over cooperation to the ahistorical "great man" narrative of historical events. This shouldn't be surprising as these narratives are useful to those benefitting from the status quo by placating those who don't, much like fear of hellfire and the promise of heaven placated those caught at the wrong end of medieval Europe's "divine right"-based feudal system (up to a point).

Our culture is very much male-centric (patriarchy is often misunderstood - even by some so-called feminists - to mean that all men are given power over all women but that's literally why intersectionality became a thing before being misrepresented in "oppression olympics" memes, so I'll avoid overloaded terms like those here). This goes hand-in-hand with the "traditional" perspective that the man/father is the head of the household and should rule it with determination and "tough love" the same way the state should lead the people (and the president the state), each family representing a scale model of the dynamics of society at large, justifying the authority of the state in the authority of the father and vice versa.

So youth and feminity in this case acts as a stand-in for submissiveness. Under the "loving care" of a controlling father figure, a youthful woman is sexually pure/innocent ("uncorrupted") and meek/submissive. By evoking signifiers of childhood (e.g. the quintessential "cheerleader" costume, braces, pigtails, lispy speech, lollipops, pastels/pink) this is shifted further into an implausibly childlike innocence and paired with the sexual allure of "corrupting" that innocence (the fantasy of "defloration" leaving a "permanent mark") based on the implicit understanding that the sexual act empowers the penetrating man and permanently devalues the penetrated "girl" lest she remains faithful to the man should he want to "keep" her. Note that we don't even need to adopt the "sex-negative" feminist perspective on penetrative sex as inherently humiliating, the idea of penetrating = empowering and penetrated = disempowered is almost omnipresent in our culture as it is (note that this has nothing to do with passivity - receiving oral sex for example is seen as empowering - and arguably the framing around literal "penetration" alone is imprecise as e.g. right-wing attitudes towards cunnilingus as being emasculating for a male "giver" show).

If all this cultural analysis is too wishy-washy for you, historical records still don't align with the idea that fetishization of female youth is innate. Young adults, i.e. women in their 20s or very early 30s, yes, sure, but not "sweet 16" or "barely legal". Arguably US culture has even gotten better in this over my lifetime given that we went from the early Britney Spears school uniform sexualization to Megan Thee Stallion and with the crackdown on public forums like the `r/jailbait` Subreddit, but there is still a very strong undercurrent, especially among conservative men.

> then what's with the pronouns

It's been the case that when I encounter people with non-normative pronouns they're trans, but you're right that isn't necessarily the case. My mistake!

I know I asked the initial question, but I guess I'm confused what exactly this conversation is about. Is the idea that people are only ever attracted to sixteen year olds because they learned to be? That feels like a challenging thing to demonstrate in the same way it being "in the genes" is, but perhaps I'm being overly reductive.

Nature vs nurture is not an either-or. I'm not saying "it" isn't "in the genes". I'm saying it's not just genes.

There's a wide range of possible age brackets, body types etc across all genders that can manifest traits most people would find attractive. Post-pubescent girls arguably aren't special in that sense. Especially if you don't isolate them out of their real-world context (which is where it stops being Oscar-winning Hollywood cinema and starts being child sexual abuse) that allows objectifying and dehumanizing them as "jailbait".

Where culture comes in is meaning. Taken at face value, a kid is just a kid. But culturally a kid represents something - naivety, hope, innocence, inexperience, whatever. This turns female youth into a fetish - something imbued with additional meaning. It's not actually the literal youthfulness that is culturally attractive in women (or else most people wouldn't react so violently against the idea of people sexually abusing minors), it's what that youthfulness represents. It's a male power fantasy.

Again, power fantasies aren't inherently a problem. What I'm arguing is that this one very much is a problem because it's so normalized it informs real-world social dynamics, i.e. where people start to forget it's a fantasy. Also I would argue the need for this specific fantasy is also not inherent (i.e. maleness does not inherently create a desire for absolute dominance over others). But I've rambled enough as it is.