I agree that the exposure to female role models is key. At the Stanford Graduate School of Business, we introduced a short course about 6 years ago called "Entrepreneurship from the Perspective of Women." Each year, over a 2 week period, we bring in 25 women entrepreneurs and investors. The impact has been meaningful; in <5 years, women have gone from representing just 12% of the entrepreneurs in a given class to over 25% (remember, women are only 40% of the entire class).
YC can do better than this. Anyone from YC out reading this? I'll even offer to help you!