When to switch between formal and informal Japanese?

Question:

I'm studying abroad in Japan at the moment and knowing when to go from "shimasu" and "ikimasu" to "suru" and "iku" is giving me some trouble. When I was in Japan last year for five weeks, I was told my Japanese was too polite, that I was too polite when speaking to friends. This year, I've gone to the opposite extreme. When speaking to fellow students who I want to be friends with, I switch to informal form almost immediately after introductions are done.

Sometimes, the other Japanese person switches very quickly also and uses plain form all the time with me. Others insist on using polite and I pick up on that and switch back to polite. Still others switch back forth: polite one sentence, informal the next, back and forth. My question is: am I switching too early? I know last year, the two most popular exchange students ONLY spoke informal Japanese no matter what, it worked very well for them.

On another note: tomorrow I'm going to kaiten-zushi with two Japanese girls. They're friends, I met them both on the same day, and have known them for the same amount of time. However, one used only polite form in text messages whereas the other one used only informal Japanese. How do I talk to them during kaiten-zushi? Do I just switch back and forth between the two forms depending on which one I'm talking to?

Answer:

There is one simple rule.

When talking to someone older than you: formal
When talking to someone younger or the same age: informal

>How do I talk to them during kaiten-zushi?

Apply the above rule.