Age discrimination in employment has received mounting attention over the past two decades, and from various cross-cutting social science disciplines. Findings from survey and experimental analyses have revealed the pervasiveness of ageist stereotypes, while aggregate and life course analyses suggest trends toward downward occupational mobility for ageing workers, especially in the face of economic restructuring and global economic pressures. In this article, we extend the literature by offering explicit theoretical conceptualization of age discrimination in employment--conceptualization that builds on prior social closure perspectives dealing with social stratification, broadly--and then analyzing unique quantitative and qualitative data on verified cases of workplace age discrimination occurring between 1988 and 2003. Our analyses center on 1.) the interactional nature of workplace age discrimination and its relation to status, 2.) how explicit ageist stereotypes both invoke discrimination and help gatekeepers justify such behavior, and 3.) the ways in which supposedly age-neutral ideologies, centering specifically on corporate costs and well-being, may also spur ageist discriminatory treatment. Results reveal vulnerability for skilled and semi-skilled workers, particularly those nearing 50 years old and retirement. Qualitative immersion into a subsample of cases reveals precisely how stereotypes are used, how employers rationalize discrimination by invoking business costs, and how the workers are affected. We conclude by discussing the implications of our findings as well as the utility of social closure theory for future research and theorizing on age discrimination and inequality. (Contains 4 figures and 4 notes.)