Other forms: ageists
An ageist is someone who discriminates against people because of their age. It would be ageist for a company to fire an employee for being too old.
Similar to sexist or racist beliefs, being ageist means having a dislike for or bias against someone because of who they are — in this case, because of how old they happen to be. It's most common for ageists to discriminate against elderly people, or to make assumptions about their abilities based on stereotypes. Sometimes adults are also ageist when it comes to kids and teenagers, dismissing their opinions and ideas because they're young.