This study offers a comprehensive framework for evaluating the moderating effects of size and age on the relationship between family ownership and innovation. The research hypotheses are tested on a large sample of Italian firms observed over the 2010–2017 period using a zero-inflated nonlinear count model. To refine the understanding of firm heterogeneity, we employ a three-way interaction approach. Results show that the patenting gap between FFs and non-FFs is sensitive to size and age. Compared to non-FFs, FFs underperform when they are small and young, or large and mature, with no substantial differences seen in other types of firms. By referring to the Socioemotional Wealth theory, we find that the founder effect, which differs over a firm’s life, is behind a good deal of these findings.