Ageism is when a person is treated badly only because of their age. Ageism can be discrimination by one person or by a part of society as a group. Discrimination is when a person is left out or abused by somebody who does not like something about them. For example, their facial features, their height or their skin.
There have been many issues at the British broadcasting company, BBC, dealing with people saying that they have issues with ageism. Ageism is often common in the entertainment industry where what a person looks like is felt by many to be important.
There are two targets for age discrimination: old people (also called senior citizens) or very young people. Discrimination against old people might happen, for example, at job interviews. A company might hire a younger worker who is about 30 years old instead of someone who is about 50 years old.