I don't like to use the words sex and gender interchangeably in cases like that, because they...aren't synonyms, haha. If it were in dialogue, you're right, a twelve-year-old (I think they're all twelve? Thereabouts, anyway) would probably say "gender", and I'd go ahead and use it, since dialogue can be written more casually. But since it's in narration, and third-person narration at that (even if it is third-person limited), I went for "sex", since sports events aren't actually separated by gender.
But anyway, my point is, that was a deliberate choice. I'm sorry it threw you off. ^^;
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I don't like to use the words sex and gender interchangeably in cases like that, because they...aren't synonyms, haha. If it were in dialogue, you're right, a twelve-year-old (I think they're all twelve? Thereabouts, anyway) would probably say "gender", and I'd go ahead and use it, since dialogue can be written more casually. But since it's in narration, and third-person narration at that (even if it is third-person limited), I went for "sex", since sports events aren't actually separated by gender.
But anyway, my point is, that was a deliberate choice. I'm sorry it threw you off. ^^;