There are various reasons people may treat you as if you're younger. I would bet the primary reason is no matter how much you think you've grown up, those changes have to be seen outwardly for a while before people that knew you as a kid will accept it. It took until 24 or 25 before I felt people had stopped treating me with kid hands. Keep in mind as well that as people get older, younger people start blending together a bit and a 20 year old does not seem as mature from a 30-40's perspective. It's similar to how younger people often have a hard time telling apart old people's ages and they all seem "old". The final point I'd make is that swearing isn't something all adults do and is not related to maturity. It can be a sign of familiarity between certain people though. Swearing around family members is either something that comes with time or never, really depends on your parents/relatives.
Trying to push these issues in an "I'm a big boy now!" manner will generally backfire and get you treated young for longer, because it's a very childish thing to do. I am not all-knowing so I don't like giving absolute advice, but personally I think the best thing you can do is not worry so much about it and continue acting in a responsible manner. If you show yourself to be mature, others will realise and treat you as such. If they don't then either you're not as mature as you think or you should find someone who you can have a more level conversation with. It's not so bad being young, anyway, "the grass is always greener" and you will miss it when people always look at you like an adult!
There's still good stuff, but less of it and fewer amateurs sadly. The "amateurs" that are there are all promoting their OnlyFans. The year or so after the sweep was pretty clear how much they lost.