Only if you want to be polite, then use "anh" or "chị". I would use "bạn" if I think that person is around the same age as mine. Also it sounds more friendly. What we do in real life when we are not sure about the age of the person we've just met is to ask immediately "Bạn sinh năm nào?" (In which year were you born?) or "Bạn bao nhiêu tuổi?" (How old are you?), and then we adjust the pronoun accordingly. I know asking about age can be considered taboo in some culture, but it's normal in Vietnam.

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.

hey, i just have read your question, yeah, if you are not sure they are older than you, you should use anh for male, and chị for famale gender. if you think they are the same old as you better you should use bạn for both gender. dont use em if you think they are the same old as you but you are not sure how old are they. if you have questions , try write to me via skype, i will help you.

If a stranger is clearly younger, using anh or chị may sound over-polite. If they appear around your age or older, anh or chị is appropriate.