Best Answer - Chosen by the Asker
You can use "em" to call somebody if you know (or guess) he/she is younger than you as well as vice versa, call yourself if you younger than him/her
In some cases for respecting the others (example: your child's teacher, your boss, higher authorities...) DO NOT use this word even if they are indeed younger than you, just call "anh","chị"...
At last, I want to notice you: the address form is very very complex in Vietnamese, sometime we even call the opposite side completely based on feeling
