Ageism refers to the prejudices, biases, stereotypes, and oppression that affect a person or group of people solely due to their age. Anyone, regardless of age, can have age prejudices, biases, or tendencies. In the past, many people and organizations focused on how ageism affected the elderly, particularly with regard to their healthcare and overall treatment by the rest of the population. For this reason, it could be argued that ageism in America is the systemized discrimination of the elderly due to the societal belief that youthfulness is superior.
The elderly may sometimes be a party to ageism, without intending to act in that manner. and simply because ageism is so ingrained in our institutions, cultures, and societies. For example, an elderly hiring manager may hire a younger person over someone who is older because of the assumption that the latter will retire sooner or require more training and the hiring manager doesn't want to have to deal with those things.
Ageism is different than most other forms of oppression, however, because while most other oppressions have clear distinctions between who holds the institutional or societal power (e.g. white vs. non-white or LGBTQIA+ vs. cisgender, heterosexual), ageism affects both the young and the old. It is no longer accepted that ageism only affects the elderly. In fact, in recent years, there has been a push to educate society on the harms that befall younger generations because of the stereotypes, prejudices, and biases that surround them.
Again, younger persons may be a party to ageism without intending to act in that manner. For example, a millennial joking with his or her peers may make a comment about their generation being lazy or unmotivated, which is a common stereotype. Although this comment may have been meant in jest, it is a microaggression and serves to deepen the oppression felt by younger people by reinforcing these misperceptions and stereotypes.