Last week, The New Republic published a lengthy exploration of ageism in Silicon Valley, the idea that venture capitalists discriminate against older entrepreneurs and that start-ups discriminate against older job applicants. The central evidence was largely anecdotal: several well-worn quotes by prominent techies talking up the innovative nature of younger founders, the struggles of a “fortysomething” entrepreneur in Boston, the thesis of an angel investor betting on older, overlooked founders, and the sociological musings of a plastic surgeon who has seen more middle-aged men coming to him to look young as a way to help their career. Nonetheless, it was enough for VC Fred Wilson to concede that, “Yes tech is biased toward younger people.”

That one of the industry’s most prominent voices wouldn’t even bother to defend it against the charge may itself be the best evidence that the start-up world has a real problem with age discrimination. Nonetheless, we were curious to know just how young Silicon Valley founders really are; is the myth of the pimpled, hoodie-wearing 20-something really accurate? So we set out to find age data for start-up founders, aware of course that this data was unlikely to tip the scales in the ageism debate one way or the other.

The data we collected confirms that 20-something founders are quite common among those who have built billion-dollar businesses.

The challenge with measuring age is that the data is relatively hard to find. Unless a founder has given his or her age in a magazine profile, or maintains a particularly public Facebook account, it’s hard to get age data without actually surveying entrepreneurs. But there is one clue to founder age that is often publicly available: the year they received an undergraduate degree, listed on LinkedIn. We decided to use this as a proxy for the age at which the founder was 22, under the assumption that this would provide age data that was accurate within a year or two. Unfortunately, LinkedIn does not include this data in its API, so we were limited by having to do manual research. We therefore picked a small but disproportionately influential dataset to examine: the founders of private, VC-backed companies valued at $1 billion or more.

Using The Wall Street Journal’s billion-dollar-club list, the Crunchbase API, and manually searching LinkedIn profiles for year graduated, we were able to generate a list of founders for 35 of the 41 start-ups, and to determine an approximate age for the majority of them. For those without graduation year listed, we scoured other sources and filled in reported age (from media profiles and the like) where we found it. The result was estimated age data for 52 founders, 71% of the individuals on our list. This, we believe, is a more complete picture of the age of today’s top start-up founders than what has been published to date:

The average age at founding in our dataset was just over 31, and the median was 30. Today, of course, these founders are quite a bit older, with an average age just under 39, and a median of 38.

Before trying to put this data in context, several caveats are necessary. First, because this sample includes a particularly successful (and therefore non-random) group of founders, we cannot generalize from it to conclude anything about entrepreneur age, or even VC-backed entrepreneur age, more broadly. Second, and more problematically, we were only able to uncover age proxies for 71% of the founders on our list. While the data presented here provides a richer glimpse at the age of successful founders than generalizing based on anecdote, and perhaps even better than some investors’ own pattern recognition, it nonetheless runs the risk of substantial bias. It may well be that founders who don’t list the year they graduated from college on their LinkedIn profiles are older, on average, than those who do. This possibility cautions against drawing hard and fast conclusions about even the founders of billion dollar start-ups.

So what can we say? Foremost, we can conclude that 20-something entrepreneurs are very well-represented among the most successful entrepreneurs. Even if our dataset is deeply biased in their favor, we can conclude that founders under the age of 35 represent a significant proportion of founders in the billion-dollar club, and most likely the majority. To visualize that, here is a breakdown of our data including the 21 missing founders:

Think of the data above as the minimum percentage of founders in each age bracket. If all of those whose ages we couldn’t find were 35 or older when they founded their companies, 46% of the entrepreneurs on our list would still have been under 35.

Our data suggests that founders on the billion dollar list are younger than tech entrepreneurs in general. A 2008 paper looked at a random sample of more moderately successful founders (sales in excess of $1 million and 20+ employees) and found an average and median age at founding of 39, with only 31% under 35. That contrast, between tech founders in general and founders on the billion dollar list, is confirmed by an analysis similar to our own done last year by a VC, which found an average age at founding of 34 for founders of billion dollar start-ups.

Those concerned with ageism will likely suggest that the relatively young age of founders in the billion-dollar club confirms that VCs are biased toward young entrepreneurs; skeptics of the ageism charge may see the data as confirmation that 20-something founders are disproportionately likely to build extremely valuable companies. While this data can’t settle that debate, it does confirm that many of Silicon Valley’s most successful bets have been on 20-something entrepreneurs.

We did uncover some data to suggest an appreciation by VCs for age and experience, however. We used the same technique to look at the age of the current CEOs and Presidents of the companies on the billion-dollar list, and this group is significantly older. (Again we were able to find age proxies for just over 70% of cases, so the same caveats apply.) CEOs and Presidents are 42 years old on average, with a median of 42.

Some of the difference reflects the fact that founders grow older while running their companies, but it also suggests that in many cases they are being replaced with older, more experienced leaders.

By way of comparison, the average age of an incoming CEO to an S&P 500 company was 52.9 in 2010. Even when it comes to installing older leaders, VCs seem to prefer the young.