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First, I'd check on the law where you live. Writing about a child engaging in sex might legally be considered child pornography. I write non-fiction so I've never personally had to deal with this...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47972 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
First, I'd check on the law where you live. Writing about a child engaging in sex might legally be considered child pornography. I write non-fiction so I've never personally had to deal with this issue. I recall reading years ago that in the United States written words are never considered pornography for legal purposes, only pictures can be illegal. And I've heard of some cases where actresses who were over 18 played minors in porn movies, either because the actress looked young or was made up to look young, and that this is legal. So I would GUESS that just writing about a 15 year old engaging in sex, when no actual 15 year old is involved, would not be illegal in the US. But let me emphasize that I am not a lawyer and I have not investigated this. Before you tried to publish such a book I would definitely encourage you to investigate and make sure you're not going to get into legal trouble for selling child porn. Note that in general, writing about illegal activities is not a crime. There are lots of books about a brilliant detective solving a murder. I've never heard of someone being arrested for writing a murder mystery. Talking about crime is not a crime. Legal issues aside, just considering it from a literary point of view ... If you write an explicit sex scene involving an underage girl, I am quite sure that some readers will find this horrifying and disgusting and throw your book away, others will find it fun and exciting, and yet others will see it as good drama. The question is: What audience are you trying to appeal to? In general, sex scenes in fiction are problematic. For some readers, if you give an explicit, blow by blow description of a violent rape, they'll say it was boring because it doesn't include any torture. For others, if you say that two people kissed they'll be embarrassed and offended. There is no "right" level of sex that every reader will find unoffensive and at the same time find entertaining. If you're trying to maximize sales of your book, you want to search for a level that the most readers find entertaining. If your goal is to write a certain kind of story, then you have to accept that maybe you're too explicit for most readers or not explicit enough for most readers.