This study explores why and how youth unemployment increases and irregular labor persists in the 2010 South Korea’s labor market. We highlight three aspects of structural transformation and institutionalization processes: first, transformation of population structure; second, institutionalization of seniority wage system; and third, reinforcement of generational networks. The argument is that Korean firms’ cost crises have emerged as baby boomers/386 core generations (1958-1963 cohorts) reach the top of the wage range (Pay Range / Salary Bands) under the seniority principle in the 2010s, under the condition in which generational networks represented by labor unions boosted the rate of wage increase in the 2000s. We predict that, the firms facing cost crises due to the overloaded People in their 50s who receive the highest wages are more likely to reduce youth employment and increase irregular workers in order to lessen the cost crises. To test this argument, we employ fixed effects models and two stage least square models (2SLS) with an instrumental variable (IV) to analyze Workplace Panel Surveys (WPS, 2005-2017). Our analysis reveal that the higher the proportion of workers over age 50(55), the higher the level of average wage, and the bigger the size of a firm, more likely it will reduce youth employment, use outsourcing, and hire more irregular workers. This study demonstrates that the cause of exacerbating youth unemployment in the 2010s is attributable to the combination of demographic, institutional, and organization factors. The findings of this study show that the interlocking of population-seniority-generation factors can increase not only inter-generational inequality due to increases in youth unemployment and youth irregular work, but also intra-generational inequality brought by increases in early retirement and irregular senior workers in their 50s.