Acting too young

When I was a TA, one of my many problems was looking, feeling and acting too young. The students did not take me seriously and would contest many of "my" rules. If the students don't take you seriously, you can lose control of the class and that looks unprofessional. But, I just wanted to be liked. This looking "unprofessional" did not bother me at the beginning, but when I saw the student evaluations I realized I had to change and start to care about looking and acting professional. I tried dressing up a little and creating a little distance to make sure they did not take me as their buddy. I thought that if I got "serious"I was not going to be liked any more, that I was not going to be cool, but later on I saw that they still liked me. It just changed from "buddy" to "respect".