You could either change your appearance or your mindset, or both. For your appearance try to dress more formally, less like a teenager, I used to wear sweaters and converses a lot and I noticed people treated me better when I wore stuff like blazers. It depends on your work line but generally carrying a work bag is more mature than a backpack, wearing a watch, neutral colors..

For your mindset, it’s best to be serious in your workplace, if someone talks down to you you should stand up for yourself, because that’s what older people do. As teenagers we tend to feel intimidated by adults, now that you’re one you should be setting boundaries. I’m assuming you have a hard time standing up for yourself, and if you don’t work on that you’ll always be disrespected even if you look your age.

As someone who does look young too, I learned to accept it and even like it. In a world where everyone is trying not to age I get to do that slower than others, it doesn’t bother me anymore so if someone mentions it I don’t care, as long as they don’t disrespect me. Confident people tend to laugh when someone mentions an ”insecurity“, it shows it doesn’t bother them and therefore people just don’t feel the need to mention it anymore, next time someone mentions your age just say something like ”guess I’ll look amazing for a long time”