Historically, in countries such as the U.S. and South Africa, "boy" was not only a 'neutral' term for domestics but also used as a disparaging racist insult towards men of colour (especially of African descent), recalling their subservient status even after the 20th century legal emancipation (from slavery, evolved to race segregation, viz. Apartheid) and alleged infantility, and many still consider it offensive in that context to this day since it denotes that men of colour (especially of African descent) are less than men.
That's easy to answer -- they're all ways to unfairly single out people based on traits that shouldn't be used to distinguish people, ways that are now illegal in some cases -- age, race, gender.
Historically, in countries such as the U.S. and South Africa, "boy" was not only a 'neutral' term for domestics but also used as a disparaging racist insult towards men of colour (especially of African descent), recalling their subservient status even after the 20th century legal emancipation (from slavery, evolved to race segregation, viz. Apartheid) and alleged infantility, and many still consider it offensive in that context to this day since it denotes that men of colour (especially of African descent) are less than men.