I need a word...
That describes the sexual relationship between an older person and a younger, underage person. Pederasty would be perfect it weren't just m/m. I need the most generic, yet descriptive word possible.
ETA: I'm not looking for a fannish term - without exception, the existing terms are gendered. I'm happy to define a new term but I haven't the lingual expertise.
ETA: I'm not looking for a fannish term - without exception, the existing terms are gendered. I'm happy to define a new term but I haven't the lingual expertise.

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Underage sex is out because it can mean two people who are considered minors, not just one. Chan typically means m/m yes, but given how imprecisely it is used I don't see why that matters. It certainly is also used for f/f and m/f, though rarely.
"Chan" is really -- I mean as it's used in contexts like this -- an appropriation of a Japanese shorthand which itself adapted affection for children into eroticisation of children. There's no point trying to get back to the "real" meaning of chan right now because it's been firmly invented as a cross-fandom term for sex involving minors. The problem with it is that it's not specifically used to talk about adult/minor pairings. It doesn't, therefore, exclude girls but is also in places used to talk about minor/minor pairings (m or f doesn't matter). It's still under negotiation, in other words, and some people won't want to call their stories "chan".
I think you're talking about "age of consent" stories, even though these are not always explicit about their address. It fascinates me that stories which negotiate age of consent narratives are never minor/minor pairings but always adult/minor pairings. And I suspect that chan, if it ever gets pinned down, will end up being a shorthand for that, but it's too loosely thrown around right now. I don't think you're going to find anything else though, that describes what you want without straightforwardly describing it as "adult/minor" (which for me begs all the questions such a story raises but which no one who wrote one could object to) or using some thematic descriptor like "age of consent" which some people will reject as not what their story is "about".
So, my call would be "adult/minor" and I'd use it even if I wasn't happy about it -- although people will try to take you on about what constitutesd a minor in say wizarding culture or elvish culture, but you're never going to be able to describe this dynamic in a way people won't resist. Please let me know if you do find something else.
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Those were example pairings. I was merely trying to point out that intergenerational can mean any kind of intergenerational story, not specifically youth/adult.
Age of Consent is problematic for say, AU fiction, where a lack of consent laws/customs is posited. We're going to use ephebophilia as the standard warning and allow writers to include additional warnings. It's not a recognized term but it's specific and once explained, difficult to misunderstand. We're still negotiating the child/adult issue. It's not something I'm personally comfortable with, but we shall see.
Ephebophilia (adjective, ephebophilic): a paraphilia of the eligibilic/stigmatic type distinct from nepiophilia and pedophilia in that the age of the partner is postpubertal and adolescent [from Greek, ephebos, a postpubertal young person + -philia].
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I wouldn't put my stories under ephebophilia for that reason, and it's also really problematic, I think, in terms of its conception of youth as not-agent, although that's perhaps more specific.
If it's your archive they're your rules but, yeah... for what it's worth.
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How about "intergenerational"? It's what I used most generically, because it doesn't imply older-to-young desire exclusively.
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Chan (and even chanslash) have begun to be understood in a non-gendered way sometimes, although I agree there's potential for confusion.
I would probably put in separate warnings of "cross-gen" and "underage", although that's not an ideal solution . .
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If so "of age/underage" might be a possibility, although like adult/minor it has the problem that it's not the same for all countries.
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