I automatically think of 8-year-olds if it's not very clear who's being referred to.

Right. "Girl" really has at least two distinct senses, one for children and one for peers/juniors of many ages. "Guy" isn't used in the first sense, and the second sense of "boy" is more restricted. The first sense of "boy"/"girl" is the most salient one, and thus the default absent further context. I don't think the first sense needs to poison the second one. But its use in the parent comment this discussion wasn't all that innocent. (I've been attacked before, by a rather extreme feminist, for using it innocently.)