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Always keep in mind, that the youth isn't so pure and innocent, like 30 years ago. The most teens start their sexual relationships around 13 and 14 (at least in Germany where I live), some earlier,...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/37008 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Always keep in mind, that the youth isn't so pure and innocent, like 30 years ago. The most teens start their sexual relationships around 13 and 14 (at least in Germany where I live), some earlier, some later. Thoughts about the other sex and sexual desires are part of the human nature. Why denying that? If I may be so blunt: It would be strange to have a story, where no one is having sexual thoughts. Sexual intercourse and thoughts are part of the human nature and especially teens (where the body is overflowing with hormones) have the desire for that. And like the answer from Galastel said: "There's no age rating for literature". So there shouldn't be any problem in writing what you want. Small Edit to my answer: TV-Shows and Movies for teens often tend to go much more into details. So I wouldn't see any problems at all