The "grown up" look

So I'm 19, and a sophomore in college, majoring in literature. I still have two years till I graduate, but yet the professors, and everyone else around me are saying that we need to "grow up". Am I not grown up enough? I rent a house (by myself), I work 20-30 hours a week, I am currently taking 18 credits in school, I don't do drugs, or smoke, my home is extremely clean, and I pay all my bills on time without asking my family for help. So please tell me how am I not grown up? Is it just because of how I dress? Must be because I am very mature. I am sorry if wearing skinny jeans to class isn't good enough for you. But why should I dress in professional dress clothes when it is just class? Please tell me. I dress appropriate, unlike some college girls you see. Tell me how am I not grown up?