I wanted to write a harem story, but in finding a justification I found what I think is a better story. However my story starts when the character is 15 earth years old. Nothing erotic will happen until 18 . Which is my first concern there is slow burn and then their a long story before the erotic. Is there a place here for such a story? Second question in creating a calendar for this world 16 colony years old is 18 earth years old. Would that get a flag?
That won't work. A longer backstory is possible, though the story needs to reach the point where the character turns 18 before you can publish the story. In that backstory, the character must not be depicted in an erotic context, so no nudity, arousal, or hot kisses.