After spending the week with Castlight Health’s S-1 filing, one clear fact emerged: Castlight is an extraordinarily young company. Despite how long many of us have tracked the company, the S-1 notes Castlight only sold and implemented its first customer in 2010. Four years later, and the company is preparing for a $100M IPO that will value it at a rumored $2 billion. We dug into the three years of operating history and other notes about the digital health company in the S-1 to make sense of the offering. Here are five things we learned: