Looking younger when you are already young-style advice

I’m 23f and I’m aware I am young before anyone comments. I sometimes am asked if I am even 18 yet, which upsets me a little bit. People ask if my younger brother who is 21 is my older brother (he is alot taller with a beard). A week ago I was asked how old I was at a store before they pushed the store credit card spiel on me ( I believe it is 18 to open a credit card). When I turned 21 I remember no one believing me or ripping alcohol out of my hands. I also avoid teen boys/times the get out of school like the plague because I had too many ask for my # , have a crush on me, etc.

When I had a serving job alot of tables questioned if I was old enough to drink alcohol, I got alot of comments on how young I looked, one table was shocked I wasn’t in high school. Honestly these moments were very embarrassing.

Any style advice for someonewho is young but wants to look more mature?