Traditionally, "Master" is used as a form of address for a boy until he is around 7 years old, although it is occasionally used through age 13. No other title is employed until he turns 18, and the label "Mister" is appropriate.
The etiquette rule is more straightforward for females. All girls under 18 are addressed as "Miss." After that, it has become customary to identify a woman as "Ms." unless she has a personal preference or has adopted "Mrs." For both males and females, these forms of address are often saved for formal correspondence and are not used on a daily basis.