Stupid question

Okay, I should've paid attention in English class but when addressing invites when do use Ms. and when do use Miss. I have a 16 yr old cousin who I'm confused about as well as some single co-workers. What about someone who is divorced, not remarried? Also, what about 2 doctors-is it Dr. and Dr. Joe Schmoe?
TIA

I think verseit.com has all these.
I am pretty sure for teenagers use miss, but for unmarried women use ms.