Utilizing an internal migration shock, this study examines the substitutability between prime-age (ages 25-54) and marginal retirement-age (ages 55-64) workers. In response to a 1% increase in young worker inflows, relative employment of young to old workers increases by 2.4%, with approximately 56% of this effect attributed to the spillover effect of the inflows. The inflow shock triggers a reduction of 0.3% in relative earnings, indicating the elasticity of substitution of approximately 8 between these two labor cohorts. Using the local projection method, I find that the inflow shock persists for the subsequent year, with an elasticity exceeding 11. These findings underscore the potential for rapid and substantial labor substitution, even within a short time frame. Moreover, in an economy characterized by such a high degree of substitutability, an unexpected inflow of young workers can significantly decrease the utility of older workers in an area, and the decline could be further exacerbated in an aging society.