The reason to measure exits as a percentage of investments five years earlier is that it takes time for a company to progress from investment to exit. According to Dow Jones VentureSource, in 2011 the median time from initial equity funding to exit by M&A was 5.3 years and 6.5 years for IPOs. So Mr. Shane's assumption of five years is reasonable.
Here's a link to the article: http://www.forbes.com/sites/scottshane/2012/02/09/the-health-of-the-venture-capital-industry/