addressing a letter to a yound boy

I received a call today from a grandmother asking me what the age was to stop addressing envelopes to a young boy with the term "Master" like Master Johnnie Doe. At what age does one stop using "Master" and switch to "Mr."? If seems that "Miss" and "Mrs." are tied to marriage, but what is the situation with "Master" and "Mr."?
Has our society dropped the use of "Master" altogether?

[Ouisch]

In the US, a young boy becomes a "Mister" at age 13.