Rita, you are lucky to look young, but I understand where you're coming from when you say sometimes you'd like to be taken a bit more seriously, thank you very much. I can relate. I am short and slim and probably look twelve from the back (not so much from the front anymore!), and when I was twenty-three and just starting out as a high school teacher, it bugged me to no end when people mistook me for a student! One time another teacher tried to send me to the principal's office. In his defense, I was dressed casually that day and I did look about sixteen.
Over the years, I've learned to make up for my small stature by developing an aura of self-assurance. Sometimes it's real, and sometimes it's faked, but it works. A lot of it was simply learning to believe that I am every bit as capable and intelligent as anyone else in the room. When I stand up straight, look people in the eyes, smile, speak clearly and think before I answer, they don't seem to notice that I'm only five foot one.
ETA wearing frumpy clothes? What? No, you don't have to do that! But I do find it helps to dress like a grown up (as you already do, in a very stylish way!).