It is customary for younger people to address a significantly older person by his or her family name, unless the older person has indicated that first names should be used. The key is the meaning of “significantly.” If you are twenty years old, a person who is forty is a generation older, so you would start out by calling her “Ms. Brown” and she would call you “Kazuo.” But she may be informal, and say, “Please call me Jane.” If you continue to call her “Ms. Brown,” she will feel that you are somewhat unfriendly.
In business, one never uses first names with a superior, a client or customer, unless requested to do so. Furthermore, even if you are on a first name basis with someone, when you are in a group of people who are not all friends or colleagues, you should refer to that person as “Mr. Brown” or “Ms. Brown.”
It is important to point out that the Japanese “san” is much broader than “Mr.” or “Ms.” in that it is used with both first name and last names. Just as we would never use “Mr.” with a first name like Bill or Jack, we would not refer to age–mates and friends as “Mr. Brown” or “Ms. Jones.” This is an important difference between Japanese and American etiquette.