Hey there. I know exactly how you feel - I was put into karate at age 13 by my mother. When you start karate training you will highly likely be put in a class that has entry-level karate students (meaning white belts). However, your class will not just be composed of white belts, but will more than likely have a multitude of levels ranging from the starting point of a beginner class (white belt) all the way up to intermediate (blue/purple/green/brown).
Also, it is very common for a beginner class to have a whole onslaught of very experienced senseis (karate gurus) training a variety of levels of karateka (students of karate). This will help you immensely. Also, you may feel a little bit intimidated for your first few lessons, but please - stick with it! You will soon discover a whole new you and a whole new ability to focus, be creative, learn, generate more confidence in yourself (if you're lacking it, like I was at first), be compassionate towards others, and, quite simply, love life. Karate is a great martial art and you will be very satisfied within yourself once you start achieving success.
So, to sum up - Until you get up to about 2nd kyu/ 1st kyu (dependent on style) you will generally always be in a class composed of kids (12+), adults, beginners, intermediate students and senseis. Once you start attending the higher ranked classes, generally everyone above you is a sensei. So, all-in-all you have a great learning environment made up of all sorts of different levels/people!
Hope this helps.
Do-ita shimashite.