Prior research suggests that as a firm grows older many of its features change, and collectively these influence a number of aspects of its behavior. In terms of a firm’s capital structure decisions, there are several studies that document how aging firms have more assets-inplace than growth options, and so justifies their taking on more debt (e.g., Hovakimian, Opler and Titman (2001), Sundaresan, Wang and Yang (2015), etc.).1 In a different vein, other research suggests that after going public, the appropriateness of different corporate governance features for aging firms also changes. Filatotchev, Toms and Wright (2006) argue that as firms age (and particularly after their IPO), their governance (board composition) needs to change to reflect its different needs. 2 More recently, Johnson, Karpoff and Yi (2016) argue that the costs and benefits of takeover defenses change as the firm ages. They report evidence that after a firm’s IPO, the costs tend to outweigh the benefits as the firm ages and is reflected in their valuation, especially in firms that employ the most stringent defenses. Both of these studies suggest that the effect of these features on a firm’s capital structure decisions may change as the firm ages as a publicly traded firm. Given the above points, we are the first study to examine how the age of a firm since its initial public offering mediates the effects of its governance on the firm’s capital structure choices. However, to examine this issue we must confront several issues that are poorly addressed in prior research.