The fact is that there's nothing unique about any age group, and the only difference between them is their birth dates. Intragenerational differences are always larger than intergenerational differences. In any age group, there are varying degrees of competence, varying degrees of attitude, varying degrees of personal circumstances (e.g., home, family).

Ageism is treating a person solely as member of a group rather than as an individual. You simply can't accurately generalize based on a job candidate's age. Anyone who claims, "In my experience [members of age group] are mostly like this..." is putting forth weak anecdotal data that is practically useless and totally unscientific.