If we really want to wank it, she could just be a teenager that looks younger than she is. It happens in real life--I'm nearly 17, and still sometimes get mistaken for around 13. Apart from the last panel, she looks around 13-ish. Her clothes also seem to indicate she's at least that age--they don't really seem to be something a kid would wear.
I haven't read the story, so I couldn't tell you how old she was portrayed in the rest of that. But having the smarts and the guts to hold a crook up with a fake gun doesn't strike me as something anyone under 11 or so would be able to do (even though it is Babs, who is awesome). The "guns are bad" line comes off as childish, but maybe Batman's just that intimidating :)
Metatextually, yeah, she probably got de-aged, especially since Kreisberg seems to like that trope.