We've seen a continuing cool trend to call folks this [something] Boy and that [placeholder] Girl for decades now. I used to be called Young Boy by kd lang back in the '80s (it's a long-ish story).
I'm sure the Fall Out Boys and the Obama Girls will be a recurring hipster affectation in the future. And I think Boy/Girl codenames are better for the Legion than the ones that ride the trend of the moment and become dated once another trend takes over.
I've always liked the idea that the Legion (some members more than others) are nostalgic for the past. It helps explain their odd fashion choices when they first went to visit Clark in the past (note Lightning Boy's pleated trousers).