This is an area of English where it's difficult to give a definitive answer - the situation has been changing over time, and the answer will vary a little in different regions and contexts. Some points:
Historically, 'Miss' would have been the correct answer - it was not merely for young girls, but for any unmarried female.
Since the advent of modern feminism (c. the sexual revolution in the 1960s-1970s), the attitude to titles indicating marital status has changed somewhat (but not universally - in a conservative area, the traditional view may prevail) and now you are 'safer' (from causing offense to feminists) to use 'Ms' as the default (for both single, married, separated, divorced, partnered, unpartnered etc. of any age). Although there may be regional variations, I believe the most widely accepted pronunciation for this is Miz. (corroborated by dictionary.com) Any regional variations would be over the vowel sound (eg Muz) - excluding Missus as a possible pronunciation. Some people find any use of a title to be patronising - it is often best to ascertain an individual's preference in how they wish to be addressed as manners can vary quite widely in this regard.
Further information: in using a title, you should be using either the surname or full name. eg. in referring to Jeane Smith the middle-aged spinster in conversation use either Miz Smith or Miz Jeane Smith - the latter more for explicit identification (first mention), the former when the context is clear. When addressing directly, Miz Smith (no given name), Ma'am or Madam depending on degree of formality or locality.