Is the word "Master" still used when addressing envelopes to juvenile boys?
Generally, for a boy age 12 or younger, an envelope can be addressed with the title "Master" but it is becoming less popular. "Master as an honorific when addressing boys is considered old fashioned outside conservative circles"
Depending on the correspondence, it's also fine to be informal, forego the title, and use just first and last name of the boy or use Mr.
However, Nancy Tuckerman (The Amy Vanderbilt Complete Book of Etiquette) states that an envelope for a boy can be addressed as Master until age 8, then becomes just his name (i.e. John Smith) until age 18 when he takes on the title, Mr. (Tuckerman, 1995).