This paper explores the differentiated effects of corporate tax changes based on firm characteristics and evaluates the potential impact of a tax system modulated by both firm size and age. Using tax rate variations across U.S. states and comparing adjacent counties across state borders, we find that corporate taxes significantly reduce employment in small and young firms, while having no notable impact on large and older firms. We then develop a model to analyze firm dynamics throughout their life cycle under different tax regimes. Our simulations show that a corporate tax system adjusted by both firm size and age is more effective than one based solely on size (and even more so than a system with a single rate). This approach lightens the tax burden on highly productive young firms and shifts it toward less productive older firms, ultimately boosting employment and welfare without reducing the fiscal surplus.