Real Dictionary

boy
countable noun: A boy is a child who will grow up to be a man.
countable noun: You can refer to a young man as a boy, especially when talking about relationships between boys and girls.

boy in American English
a male child from birth to the age of physical maturity; lad; youth
interjection: an exclamation of wonder, approval, etc., or of displeasure or contempt
noun: a male child, from birth to full growth, esp. one less than 18 years of age

boy in British English
noun: a male child; lad; youth