The Mishnah before us in Maseches Nedarim deals with someone who vowed not to benefit from "shechorei harosh" - "the black-headed ones" - and with the question of who is included in this wording of the vow and who is not.
Who is included in the vow:
Bald men - although the expression "black-headed ones" seems to refer to someone who has black hair, a bald man who has no hair at all is also included in the vow.
Men with white hair - they too are included in the vow.
The reason for this: the one who took the vow did not vow against "those who have hair." Had he said "people who have hair," he would have included only those with hair; but the term "black-headed ones" is used as a general expression for men in general, and therefore men with white hair are included as well.
Who is not included in the vow:
Women - they are not called by this name, since their practice is to keep their heads covered at all times, and they are called "the covered-headed ones."
Minors - young children do not keep their heads covered at all, but go about bareheaded, and therefore they are called "the uncovered-headed ones."
It emerges that the expression "black-headed ones" refers specifically to adult men: since their heads are sometimes covered and sometimes uncovered, and most of them have dark hair, they are called by this name. And once the name was given according to the majority, it includes as well those whose hair is white and those who have no hair at all.
In summary: one who vows against the black-headed ones is forbidden to benefit from bald men and from men with white hair, and is permitted with women and minors - since this name is given to men alone, while women are called the covered-headed ones and minors the uncovered-headed ones.