Ageism is the discrimination against individiauls or groups of people based upon their age. This can manifest itself in systemic discrimination, prejudice, or stereotypes people may have of people based on their age. One salient example of ageism is how older folks are portrayed in TV and movies - cranky, forgetful, comedic relief, always side characters, usually men.
Nelson, T. D. (Ed.). (2004). Ageism: Stereotyping and prejudice against older persons. MIT press.