I was addressing the (to me) simple and straightforward question of whether there is any moral horizon crossed in a 20 year old catching feelings for a 15 year old friend/girlfriend of a friend. There is precisely nothing inherently unbecoming about that.
The problem here is that your "catching feelings" framing of this situation is not really adequate to describe what is going on here. He didn't watch her and admire her from affair; he cultivated a deeply inappropriate relationship with her, and exhibited significant hallmarks of an adult grooming a child. His conduct was quite a ways beyond "unbecoming", as even he expressly says. To even have those feelings for a girl that young is creepy enough, frankly, but you add in the fact that he is actively seeking her to appreciate how much he loved her....Tres...this was never going anywhere good and it's just plain disturbing.
As to the other matters you spoke to, of neurological development and whatnot, I wasn't speaking to that. On that subject, though, I'll reiterate what I said previously: You can very, very easily -- whether it "should" be the case or no -- have two young adults five years apart in age yet still of comparable emotional range and development.
As you pointed out, that chunk of time is potentially one of vast flux. However, it is even more easily one of prolonged stagnancy. What is put into a system is largely also what comes out of it.
As I went over before, hardware (i.e. neurology) is a limited part of the equation. Software matters just as much, and I'd be very hard-pressed to be convinced that your typical 20 year old sees much difference in their software from what they saw at 17 or even 15.
Again, don't take hardware out of this, but don't treat it as the benchmark either. In all practical senses, developed countries have a culturally delayed adulthood from what was recognized in decades past, due to multiple factors (largely economic, unfortunately). People are usually bound to their parents/guardians for longer than was ever expected to be the case, and as such, their range of experience/development continues being largely a product of that filter.
Kids given the leeway to be rebellious at 15, intent on not divulging where they are or what they're doing (to assert their "independence"), are still carrying on such behaviors at 20. Kids who have strict controls imposed them on 15 are still dealing with those at 20. And kids with parents who have simultaneously cultivated agency and responsibility are still engaging with that. It is no longer even uncommon for "kids" to not be setting out until their mid- to late-20s.
Nothing is changing for young adults for an enormous span of time. That's the world we have built, so that's the world we live in.
What you are vaguely riffing on here is Skinnerism/radical behaviourism. It's an antiquated school of thought that has not held up well in the light of modern experimentation. The notion of the blank slate is not compatible with the modern brain science. I actually just deleted a sizable little side discussion exploring the topic of of natvism vs. behaviouralism: if you have an interest in theory of mind, I'd be thrilled to have that conversation with you at some point, but a detailed divergence into how the cortex develops in late adolescence isn't really the best way to discuss this.
So, look, I will grant you this as an alternative way of approaching the topic: a 20-year-old brain is by no means a fully developed brain: neuroplasticity finally ebbs and peak developmental potential is reached for most people around the age of 25. That is so. But this does not change the fact that a 15-year-old has far less capacity than a 20 year old--they just do. And the problem with the whole "well there's some variability" argument, is that his is exactly part of the standard rationalization of child abusers around the world: they mostly tend to think of their relationships as love stories. Society cannot afford to tolerate that kind of exceptionalist thinking in this area; we just cannot have a "Nah, it's cool, she's mature for her age." exception to pedophilic impulses. And a man being 20 years old and technically less put together than a 25 year old does not abrogate him of the requirement not to exploit a child. Whoevever likes it or not, most developed nations have set a standard age of 18 or so as the age at which you are deemed to have full capacity to comport yourself with integrity, and after which you are subject to the full force of the law when you do not. And no amount of "well, kids are being coddled today, so it's no wonder some of them don't learn responsibility" reasoning changes the age of consent, or the basic facts of the inherently exploitative interactions it prohibits.
Children need to be protected from those who don't get this. And in the case where only feelings are confessed and then the underage party wisely cuts off communication, with regard to the adult, we are clearly still talking about someone who doesn't get that. We shouldn't sit around on our hands giving him the benefit of the doubt as to just how much he doesn't get it. And we certainly shouldn't be saying anything that might suggest to children/former victims in this area that they shouldn't be speaking up if the man never touched them... I just don't see any reason why, an adult having conducted themselves in that way, it should be considered taboo or "excessive" or distasteful for the child in that scenario to speak up, even years later when they have become an adult, had time to process and gather the courage to speak up fro themselves. We clearly need that shit, both to remind us how pervasive the problem still is, demonstrate the human cost, and identify the dodgy mofos we need to be keeping an eye on.
Now I said I'd stick away from the broader neuroscience of the areas you touched on, but I've found a way to wedge some in here in a way that is quite on point: you were saying that you felt that experience/environment was the prime motivator here and that as a consequence of that, since the lives of adults between 15 and 20 are largely rote (not necessarily the case, but let's pretend it is for the moment its a controlled-for variable), and thus as a consequence they really wouldn't be acting all that differently. But that's not really how it works: the development of the brain is genetically programmed, and it's proceeding in a general anatomical fashion in large similar fashion somewhat regardless of what the individual learns during that time.
So just because a given person has an upbringing that you (or I, or person X) might arbitrarily define as "stunted" and not likely to promote responsibility doesn't mean that person's brain is developing less efficiently: it just means they are learning different stuff. Which yes, may or may not make them a more or less responsible (or flexible, or dogmatic, or truthful, or capricious) person in some respects. But much of the brain's development (and the effects upon behaviour merely as a result of certain brain regions getting boosts at certain points in development, rather than discrete neurons encoding specific lessons) proceeds according to how it is genetically programmed to develop. I'm simplifying here somewhat in that there are numerous areas in which developmental neurophysiology is impacted by epigenetic effects (the manner by which environmental factors influence gene expression), but the general principle stands: just because we might think of someone as having a less favourable learning environment does not mean they have an altered rate of brain development.
However, there are always big exceptions to general rules in this area and you want to take a guess at what one of the biggest factors is that actually does tend to seriously stunt development and/or cause longterm detrimental effects in how behaviour develops? You guessed it, sexual and physical abuse.
