You don't need a PhD to understand that young people (what we call millennials this generation) are easy to manipulate. Their lack of experience makes then ripe to work for startups. Hiring 'older' workers will only bring more challenges since they are more likely to question and challenge false idols. There are exceptions of course, having older folks who also believe the hype and feed it to the younger kids works great to produce the startup echo chamber. Once the leadership is in place you want to recruit people who are more likely to drink the Kool aid. This is not rocket science. A prime example is the military.