For all the hype about the baby-faced billionaire—from Bill Gates and Steve Jobs through Mark Zuckerberg and Andrew Mason—startup owners have a bit more, well, seasoning.

That’s one result of a study published today by LinkedIn and its senior research scientist, Monica Rogati. She found that 65 percent of startup founders are 30 and older, with 40 percent of them nestled together in their 30s. The second-largest group, 34 percent, were in their 20s.

Rogati sorted the 120 million public profiles on LinkedIn's network system and sifted for founders of companies to come up with what she’s calling the entrepreneurial genome. Some of what she found isn’t all that surprising:

• Stanford, Harvard, and MIT Sloan business schools were the top entrepreneur-producing business programs.
• California, New York, and Massachusetts were the hottest states for startups.
• People with networks that included venture capitalists, online publishing, Internet work, and recruiting were most likely to found businesses.
• Alumni of companies like Apple, eBay, Yahoo, and Google are more likely to found their own companies than were denizens of the rest of corporate culture.

But some of the results were at least a little counterintuitive to those who think entrepreneurs emerge fully formed from college and launch their companies immediately before going on to create firm after firm. Sure, there are such entrepreneurs, but:

• Entrepreneurs in their 30s make the fat part of the bell curve. Of course, when you combine those in their 20s and 30s, that makes up a huge 74 percent majority of company founders (at least those with LinkedIn profiles).
• The average length of time in a given position was 2.5 years before a founder started his or her company.
• Just 2 percent of founders are serial entrepreneurs. That means most founders are faithful to the company they started in the first place, or flunk out of owning their own business altogether.

Here’s a key takeaway from both the education and the kinds of networks founders have before launching, Rogati writes:

Relationships and mentors are crucial to entrepreneurs’ success, and this is reflected in the connections they have on LinkedIn. Founders are disproportionately connected to venture capitalists, bloggers, and recruiters.

Now, that may be a shameless plug for a social-network scientist to make. But it also happens to be true.

Get more business intelligence from Portfolio.com:

• IPOs Dry Up in August: More companies backed off going public in August than have in any month since the dotcom crash of 2001. The drying up of public offerings could have been caused by market volatility, but some see a closing of the window for hot startups.
• Surfboards, Antlers, Fees: Now that airlines have to post a list of fees and surcharges, we know more about the little extras that they try to tack on to our fares.
• Sun Sets on Solyndra: The solar company was the first recipient of loans backed by the government to juice alternative energy. But Solyndra still couldn't compete with lower-cost rivals, and now it's bankrupt.