Hi Jazz, When you talk with stranger and want more conversation with him/her, but you are not sure how old he/she is, you can call "Ban". To prevent this awkwark, you might ask for her name or you can start introducing yourself first (such as I'm a student or has been worked...for 3-4 years) than she can adjust the personal pronoun. Or if he/she just crosses by you, use "anh/chi" is ok, no need to wonder alot. "Anh/Chi" is used in a formal, polite conversation, especially in the customer service. For example, when you enters a restaurant, a waiter/waitress will always call you "anh" even he/she is obviously older than you. (the same for shopkeeper, sale man,...)Calling a waiter "Ban" at this situation is a little bit childish.