You might hypothetically be able to question that judgment, actually. I don't know which version of Supergirl you drew, but I know in Superman: The Animated Series, she was 16. In Justice League Unlimited, she references that series as happening a few years ago. According to Bruce Timm, that puts her at 18 to 19 years old, and in some stories, she's in her 20's- 21, in fact.
dA's rules state: "Characters which have been portrayed as a range of ages in their original media will be classified as underage or not based upon the youngest portrayal as well as their average age over the series."
To do a quick average (16 + 17 + 18 +19 +20 +21)/6 = 18.5 years old... which by dA's average age rule seems to imply she'd be OK.