My step mother decided Im not an adult at 21?

You have to be above that remark and it sounds like you are a responsible person. You may never get any positive results from this woman and that's ok, just change your attitude and know that you are doing the best you can. I know it is hard, contribute all you can to the household, do your best and know that is what you are doing. I know it can be very frustrating, but never, ever let anyone else make you feel any less of yourself. In reality, she may not have meant it in a rude way, she may look at you as a young person, not an adult because if she is over 30 or 40 you do seem young to her. Alot of times people don't realize what they are saying and at 21, I know I was touchy about certain things. Not that anything is wrong with that, but I wanted to be acknowledged for what I contributed as well. That will come when you are older, trust me. Saying that, just maybe remind her in a gentle way that you do have feelings and they do get hurt and you would like some positive remarks about the good things that you are doing. Good luck to you , I hope it all works out.