Ageism refers to stereotyping and generalising on the basis of age, typically old age, and is spreading to middle age as the boundaries of middle age move into what was once considered old age. (Brannen)
“Stereotyping and generalizing about people on the basis of their age; commonly, a form of discrimination against older people.” (Barker, 1999)
“Applies especially in such actions directed against older people, but the term may also be employed to refer to unreasonable stereotyping or discrimination against anyone where this occurs simply by virtue of age.” (Jary & Jary, 1991)
“theorisations of later life that have, almost exclusively, concentrated on British stereotypes of old age” (Wray, 2003, & Phillipson, 1998).

As defined by Brannen. Barker, Robert L. (Ed.). (1999). The social work dictionary (4th ed.). Washington, DC: NASW Press. Jary, D., Jary, J. (Eds.). (1991). The Harper Collins dictionary of sociology. New York, NY: Harper Perennial. Phillipson, C. (1998). Reconstructing old age: New agendas in social theory and practice. London: Sage. Wray, S. (2003). Women growing older: Agency, ethnicity and culture. Sociology, 3, 512-527.