When to Use First Names

From As a Gentleman Would Say by John Bridges and Bryan Curtis

Although the world today is for the most part on a first-name basis, a gentleman knows it is always safe to address a new acquaintance as "Mr." or "Ms."  He especially follows this rule of thumb if the acquaintance is an older person or if he is dealing with his superior in a business environment.  However, once Ms. Jones has told him, "Please call me Mary," a gentleman concedes to her wish.  Otherwise, he runs the risk of making her feel ill at ease.  As a general rule, if a gentleman finds that a peer is referring to a person as "Mr. Brown" or "Ms. Smith", he may logically assume that that person wishes to be referred to as "Mr." or "Ms."  He does not force business acquaintances to act as if they were his personal friends.