As another commenter pointed out, this off-hand comment is harmful to girls. In my own experience growing up, girls were just as "professional" (if that can apply to teenagers) as boys. They were serious, thoughtful, ambitious, and worked hard in school. Now I'm older and my friends have kids, and their girls are going to Maker Faires, studying hard, etc.

We should not accept the meme of a "teenage girl" being a silly thing that doesn't know how to behave right, because it's just not true.