There was a whole issue of "Brave and the Bold" by Mark Waid a couple of years back in which Kara has an obvious crush on Hal Jordan (it was GL/Supergirl team up) and he has to keep reminding himself that she's "seventeen, seventeen, seventeen". He keeps repeating that throughout the book to remind himself she's jailbait (even though she's an alien from another culture and this is the same guy who was able to get with 13-year old Arisia -who rapidly aged herself to be with him- on that reasoning). Recently in her own book Kara just celebrated a birthday so she should be 18 now.
But we are dealing with comic book ages here. It's a world where Bruce Wayne and Clark Kent are perpetually 35.