The article seems to make a fairly solid case:
"Our team analyzed the age of all business founders in the U.S. in recent years by leveraging confidential administrative data sets from the U.S. Census Bureau. We found that the average age of entrepreneurs at the time they founded their companies is 42. But the vast majority of these new businesses are likely small businesses with no intentions to grow large (for example, dry cleaners and restaurants). To focus on businesses that are closer in spirit to the prototypical high-tech startup, we used a variety of indicators: whether the firm was granted a patent, received VC investment, or operated in an industry that employs a high fraction of STEM workers. We also focused on the location of the firm, in particular whether it was in an entrepreneurial hub such as Silicon Valley. In general, these finer-grained analyses do not modify the main conclusion: The average age of high-tech founders falls in the early forties."
What's the counter-argument? That the famous founders of the 20-so most known companies are young?
There are thousands of tech companies founded, even on SV, not just the household names (and the household names are a tiny sample anyway).