Well, I would say the fact that she is three years older may indicate she hasn't had a typical path up to now, which may be part of the reason she doesn't understand basic cleanup habits.

I've realized some people do better with writing- like an email, some do better with reminder notes on a white board.

Some actually start to get the hint when you disallow them to use your stuff anymore and either ignore their dirty dishes or place them in front of their door/on their desk, etc... but that has only been after having explicit conversations.

Sometimes just explaining expectations in a meeting where everyone agrees on what ar appropriate expectations- kind of like startind over with an "apartment plan" on how to deal with dishes, chores, etc... could help. If there is no finger pointing but more "so how do we address cleaning up the kitchen on a consistent basis?" "how do we adress the fruit fly problem [which you may or may not have because of food being left out]" things like that... addressing the issue as a shared problem to sort out rather than "this is what you are doing wrong/that is upsetting us"

Um. Sorry, it's late.
She may not have had a lot of responsiblity before now and is simply not used to doing any of the things that are expected of her.
I don't know if y'all are cleaning up after her, eventually, but if you are, it's not going to help to demonstrate there are consequences.

Roommates are difficult, in general. Sorry you are having such conflict.

ETA: I've also found that, sometimes, pointing out hwo unproductive a person's communication style is can be helpful.

As in: "Everytime we want to discuss an issue in the apartment there is a lot of yelling and defensive talk and it gets extremely tense, how else could we ALL address these issues?"

I mean, you don't want to have to hand hold, but you kind of have to be a guide if you want a decent roommate sometimes.