To a mom, their daughter/son are child forever. I mean she gave birth to you, took care of you, breastfed you, changed your diapers (lol). You'll be a baby to her forever, no matter how old you grow. Even when you're 50 you'll be a baby to her.
But I agree it gets tiring when my mom doesn't take me seriously. I want to be taken seriously. I want her to know that I'm responsible enough that she doesn't need to worry about me.
My suggestion would be to talk to her about it. Tell her how you feel. Tell her you're growing up. That you're not a baby anymore. But speak to her calmly, don't get mad about it. Getting mad or whining will only make you seem immature. And prove to her that you've grown up. Show that you're responsible. Completing your schoolwork on time, taking care of yourself, keeping your bedroom and surroundings clean and organized will show her your maturity.
I want to add something else, our mothers aren't immortal. As much as it saddens me to say this, but my mom won't be with me forever. Your mom won't be with you forever. So let's try to cherish the moments we have with them, and make lots of great memories. :)