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"New Adult" was a category that St. Martin's attempted to launch 10 ish years ago - coming-of-age books about the 18-28 ish age range. That was the main parameter, like YA with teen mc's. But because bookstores had no place on the shelf and didn't want to make one, 1/ https://twitter.com/carlywatters/status/1387432632569454600
They'd take the New Adult books being experimentally published and shelve them in romance or YA, or just plain Adult depending on the genre. As a result, NA got a reputation for being mainly college romance. So when agents say they're not looking for New Adult, 2/
It's because publishers "aren't publishing it" (at least with that categorization) because bookstores won't shelve it. Books with mc's in that age range now go to Adult or get aged down to teen for YA, so that they have a chance of being shelved somewhat properly, or at all.
When we say we're not looking for NA, it's because poor planning and an inability to shelve it killed it. If you're writing about older college-aged mc's up to late twenties, safer to just call it Adult in your query.
I will add, NA does well online because methods of categorization differ from how physical bookstores shelve books. So there's a reason you see it actually used in self-publishing circles or webcomics, etc.
