Ageism refers to both the stereotyping and discrimination against people due to their age. Whether the person is younger, older, or any age at all, they can suffer from ageism in the workplace. Ageism can be supported by policies that reinforce negative stereotypes. Others in the workplace may have prejudices against staff of certain ages, causing them to act in a discriminatory manner.
There is legislation against discrimination based on age, so you cannot use this as a factor in the hiring process, for example. However, it can show itself in other ways, such as passing over someone for a promotion.