Prejudice against others because of their age especially prejudice against older adults is known as

   Ageism

Explanation:  This is, therefore, prejudice, or better yet, discrimination against people based on their age, especially when it comes to older people. This attitude comes from certain stereotypes, so this phenomenon is not the result of objectivity but of stereotyping. This ageism can also be fatal for older  people, because often older people are marginalised in every way, from employment, through social services to neglect and total exclusion from the community. Although this attitude towards the elderly is completely defeating, ageism is somewhere even a normal occurrence like all other types of racism on any other basis, unfortunately. Older people who are exposed to ageism can often have devastating health problems as well as problems in their day-to-day functioning that are a real struggle for them.