Childish Behaviour

I'm twenty-three. And I like to think I act older than that. I handle things in a mature, adult way. Or I like to think so anyway.
For the last week, I was at J's for part of my Spring Break. Which was awesome, I missed him so much and was glad to have more than just two days with him. We found lots of stuff to do, and went out and spent time with his friends one evening, then my friends last evening.
Anyway, last year when I lived with him, we bought a boxed cake mix. But never used it. Last time I was there, I noticed that same cake mix in the cupboard when we were organizing. It had a best before date of November 2011, but we decided to keep it anyway. But that weekend was super busy so we didn't get around to it. So we saved it until this week. While I was there, I asked J if I could make the cake. Since I didn't want somewhat good food to go to waste. Cake mixes can get expensive you know.

So I made the cake yesterday. I only had enough time to take it out of the oven and let it cool before J and I left for some errands, and to meet up with friends for dinner. Turns out I made a mistake when I made the cake, because I used one of J's roommate's cake pans to do so. And didn't clean it right away, although I did have the intention of cleaning it later when I did get home.
And this roommate can get very territorial over things like that. I may have written a rant about her before when I was living with them at the time. But anyway, she texted J while we were out with friends, bitching at him for using the cake pan.
J decided to ignore it.
Then as we were walking home after dinner, he said he wouldn't be surprised if the cake pan had been cleaned by her and hidden by the time we got home.
Well, turns out he was right. When we got home, the cake pan was no where in sight.

This sort of annoys me. This roommate of his is 36. She's an adult, but she's acting like a 3 year old who doesn't want to share with anybody. My parents definitely taught me the value of sharing by age 3 I'm pretty sure.

I mean, hiding something like that so nobody but her can use it? It's ridiculous.
I'm thirteen years younger than this person, and I act a whole lot older than she does.
Childish behaviour from adults does not sit well with me.

If she didn't want her cake pan to be used, she could've said so nicely when we got home. I wouldn't have minded. I would have said sorry for overstepping her boundaries by that point. But since she hid the damn thing, well, I don't think she would've wanted me to say sorry. So I didn't. I actually ended up avoiding her the rest of my stay to avoid a temper tantrum.
Which is just as well, because even though I'm a calm person that doesn't like a lot of ruckus, I probably would have given her a piece of my mind by that point. And that wouldn't help anybody, since J would be stuck living with her after I came back here.

I guess I'll never be baking at J's house anymore. Which is unfortunate, since I like to bake, and it goes get hard to bake a lot of delicious treats for just myself, because then I end up having to eat it all by myself.
With the exception of missing J like crazy, I have to admit I'm glad I moved away. I now have the freedom to cook in my own kitchen, comfortably ...although I should have been able to do so last year as well. But I digress...

I really don`t know how to approach this situation. J already knows how I feel about this roommate of his, and agrees that she acts childish and selfish, but doesn't want to confront her about it, because it would get worse. If that's possible. But how is someone supposed to learn how to behave more adult, if nobody tells her about it?