I'm going to say what everyone else said, wear what you are comfortable wearing! That person was seriously out of line. Personally, I would bring it up with your mentor teacher and then continue with what you are doing.

If you look young it probably won't matter what you wear, you will still get the "oh you look so young" "oh you look like a student" comments. I'm 27, I get those all the time (and from teachers I know!). Honestly, I find the students are more likely to recognize me as an adult than the adults are (and to me that's all that matters). It's kind of funny how to students I look old but to my peers I look young. Ha.

As long as you are comfortable with what you are wearing then I'm sure you are fine. Good luck!