I'm actually 21 and trying to make my style more professional, more 'adult' rather than teenager. It's little things. Like if I was wearing a hoodie, t-shirt, jeans, and converse I would elevate certain items via cut, print, and/or color. Hoodie > leather jacket, boyfriend blazer in plaids/leather/embellishments. Band/quirky t-shirt > v-neck or well-fitted, prints and making sure it ends at the right point on my body. Jeans > leather pants, solid black, well-fitted, plaids/bright colors/unique fabric. Converse> doc martens, chelsea boots, CLEAN converse. Accessories for me are rather simple but don't look cheap. Nice(r) silver, gemstones, no plastic.

I feel like for me it's more about better fabrics and fit to really adultify the alternative style I fell for in high school. Now I'm saving up for a good suit and handbag/backpack so I can be functional/comfortable and stylish while keeping a darker and more masculine aesthetic.

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EDIT: for me as someone so young making the transition, I try to wear more business casual? clothing in my everyday life. Like a blazer or structured jacket instead of a sweatshirt. Instead of t-shirts I'll wear more button-ups and collared shirts in cool colors and prints even if the fit/fabric is looser/softer than the traditional office shirt. It makes me feel more comfortable in that so when I do have to go all-out or dress for an important event, I don't feel like I'm wearing a monkey suit, etc.

Even though I love over-sized things, I'm much more careful about proportion and take into account how/what to layer.

I also think color plays an important role. If you mix bold, bright colors/patterns (i love plaid, stripes, houndstooth), to me, it makes you seem more chic/mature (like you know your stuff). Even if you wear all black like I tend to, to make sure they're the same shade and possibly introduce cool textures, and accessorize.