This is in NO WAY, any form of legal advice. However, the information below is provided for further study and research. (That is my disclaimer.)
You should think about having a "standard disclaimer", or simply do not state ages, if you think it may be an issue.
However, "creative works of art", which are fictitious in nature, are not quite bound to "mortal laws". However, you, or your readers may be limited in "what is portrayed", in some locations.
The legal age of consent, for most things, in most places, is 16 years of age. Though, while under parental-control, there is another law that extends "consent", to include the parents. (Thus a stipulation to the law, related to "statutory rape", which is not actually rape in most cases, but just a parental disagreement with any sexual activity with that "individual" and the one they have "parental-control" over. This is not "written in the law", but that is how it is handled in reality.)
In the USA, 16 is considered "legal to consent", except where the above is taken into consideration. In some states, they attempt to defend 18, but anyone who actually fights it, usually wins. (This includes individuals who are NOT "under parental-control".)
Even still, because the "acts" do actually happen to individuals of lower ages, with or without consent, they are considered "real". Some places do not like portrayal of these actions, considering them to be "lewdly pornographic", even when not actually pornographic in portrayal. In the "movie" industry, you are actually allowed to portray certain situations, with the proper "guidance and supervision", actually including the "not of legal age" individual in simulated situations. (The movie "lolita" is just one example.)
When it comes to artistic freedom, there is a strong hypocritical stance. You can simulate mass-murder of children/teens, beating children/teens, torturing children/teens, but not rape or "naturalism" (nudity). Though, most portrayals are often just given a warning, unless they are derived from the USA. They will ask you to stop distribution, or include disclaimers about "strong sexual content" or "sexual content for mature audiences". Which then leaves the "burden" on the viewer of the content, if they chose to continue viewing it, at that point.
Things to consider, when writing... To avoid this stuff...
1: Do not state ages. (You can imply age, without stating it. If you state it, you are stating it for a reason of identification. "They are adult midgets", "They are young"... looking, but of legal age.)
2: Do not state origins. (If you localize and identify with specific reality, you are doing it for the purpose of identification. "They are dreams", "They are from Mars".)
3: Do not state genetics. (This is the standard cop-out that most take. "They are aliens", "They are vampires", "They are robots".)
4: Admit to horrible depiction. (You have horrible scale, bad with numbers, poor writing skills, and/or you got the characters mixed-up. It was a type-o.)
Above all, you have to remember that these are just "thoughts", and not "actions", and it is rarely ever a crime to "think about a criminal act or portray one", only "committing actual crimes, or instructing others to do so". If the above things are not stated, as identification, then it is the person who is "questioning you", who is the only one guilty of "thinking those things". Because they are the ones "filling-in" that missing information, which is not stated.
By the way, the "most read book", by 15/16-year-old girls in America, at the moment, is still "50 shades of grey".
Further general research:
Legal age of concent