Real Dictionary

old boy
countable noun: If you refer to an old or middle-aged man as an old boy, you are referring to him in a disrespectful way.
countable noun: You can refer to a man who used to be a pupil at a particular school or university as an old boy.

old boy in American English
an alumnus, esp. of a boys' preparatory school
noun: an adult male, esp. a Southerner

old boy in British English
noun: a male ex-pupil of a school