This study uses learning theory to show how knowledge domains affect product extension decisions and how these product decisions change as firms age. Faced with the choice of new product-markets, a firm might decide to introduce a similar product, by leveraging existing firm knowledge, or to experiment with a less familiar product, which requires new knowledge. Using data on new drug introductions in the US generic pharmaceutical industry, the analyses showed clear support for heterogeneous product-market entry patterns across knowledge domains as the firm ages. Across time, the form of learning shifts from exploration to exploitation. Copyright © 2013 John Wiley & Sons, Ltd.