Ah, damn. Thanks for the extra info. I was really hoping I'd be able to leave this comment thread forgetting that he had formerly had problematic views!
(To be clear, I mean I wish he didn't have problematic views any more, not that I wish you hadn't informed me.)
Speaking of typos, I think you mean <=13 not >=? As in, referring to the fact that he previously thought it was ok with kids of any age (vs having now changed his mind about younger children but still thinking it's OK for 13+)? Unless I'm either misunderstanding the situation with him, or am making my own mistake about sentence structure or > vs < logic in some way
Edit: on the subject of why would someone need to be told that it's bad, on that point I actually don't agree with you. Because while I don't think there's an excuse for needing to be told that raping someone is bad, I have known several people who had sexual experiences when they were children, that they considered to be consensual at the time and that as adults they looked back on as not being negative, and therefore their opinion was that if a child "consents"/wants it to happen, then it's morally OK.
Of those people who I've personally discussed it with, 100% have changed their opinion after learning how it can have terrible long term effects on some people regardless of their having believed they wanted it to happen at the time.
But I don't think it's necessarily intuitive that if a 12 year old believes they want to have sex with an adult it must be wrong, especially not if, like these people I've known, they themselves had that experience and were lucky enough not to suffer in the long term (at least I hope they're all still not suffering).
It's the fact that we know from looking at the big picture that it's likely to cause problems in a child's development that teaches us that actually we shouldn't consider a child saying "I want this" to count as consent. If anything it's unintuitive, since as a rule of thumb we should think that people, including children, should have agency over their own bodies - and we make an exception in this case, because enough data has shown that children consenting to sex does, far too often, lead to mental health problems, if not immediately then later in life.
(I think/hope I've been clear enough that nobody would read my comment and get the impression that I'm condoning adults having sex with children. If any of my wording does give that impression it's a mistake. Do not have sex with children, ever.)