The idea of the wicked stepmother or evil crone is a common one in traditional fairy tales. These women are usually middle-aged or elderly, and they prey upon younger women who have supplanted them in beauty or influence. The evil queen in "Snow White and the Seven Dwarfs" might be the ultimate example of this trope, willing to murder a child once she learns she is no longer the fairest in the land.
One of the reasons for the prevalence of this trope in traditional fairy tales might be because in the societies in which these fairy tales were produced, women were often prized for their purity when virgins and their ability to bear children when married. Since the woman's role was limited to the realm of marriage and sexuality, her beauty was also a prized asset—more than intelligence, creativity, or courage.
In such a limited role, women lose their sense of worth once they lose their youthful looks or can no longer have children. As old women, they are seen as burdens. Therefore, such women become jealous of younger women who possess what they have lost. Anne Gray Harvey's 1971 retelling of the Snow White story observes that such behavior is ultimately cyclical, ending her version with the implication that the young and fair Snow White will eventually become like her vain stepmother, since beauty is the only asset men have prized her for in the tale.
It must be noted that the idea of all older women being evil in fairy tales is not a universal one, however. The archetype of the helpful crone (also called the wise woman) is also common in many fairy tales around the world. These women can be presented as fairy godmothers (as in "Cinderella") or kind grandmother figures (as in "The Snow Queen") who provide magical aid, shelter, or comfort to the heroine.