Master is a courtesy title for young boys too young to be addressed as Mister. However, in most modern social circles the term is considered archaic, and young boys are called Mister or simply not given a title.
According to Emily Post, the title Master can be used on formal correspondence to young boys until age eight. The young man becomes "Mr." when he turns eighteen. During the years between, no title is needed. An exception is if you happen to be addressing the oldest son of a Scottish viscount or baron, where Master is the proper title no matter what his age.