It's 'normal' for us teenagers to greet adults when we walk in. But the way to greet adults are different. For me, it is required to take your shoe off and when you walk in the house and greet properly but for my friends, it is okay to walk in the house with shoes on and greet casually. I find it awkward to walk in the house with shoes on while my friends are completely fine with it. I see my friends going over another friends house very comfortable and casual but for me I find it very uncomfortable because I have the anxiety that I'm intruding into their home and being a bother.
Eating is essentially the same, but the way we eat is different too. My family eat with chopsticks and all while my friends family is all about them forks!
All in all, it is how each individual is brought up and the environment they're grown accustomed to.