Answer: a Stereotype
Step-by-step explanation:
A stereotype is a belief that is held by many people about a group of people in terms of how they behave and the abilities that they have.
This belief, which is usually borne out of actual instances, is then generalized (usually wrongfully) to apply to all members of that group instead of recognizing that this may not be the case for all the people in the group.
By believing that all older people have trouble hearing, you would therefore be engaging in stereotypes.