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The story never gives a specific age for any of the characters, but they are all 18+. Clearly the parents are, but the step sister and step brother, as well as all their friends, are at minimum 18. However, maybe they're as old as 25. That is left up to your imagination. Their school is also never specified, so maybe the step siblings and their friends go to a 4 year university, a two year college program, a graduate program, or heck, this is a science fiction/new world order story where men around the world are shrinking. So, you could imagine it as some kind of alternate Earth where there's a school of only 18 year-olds, which is where the school determines their future employment. I always tell my fans that it's up to your imagination when they ask me questions that the text does not answer.