There were more in the 70s, 80s, or 90s (E.g. Jim Clark) because the industry was still forming, but even then the wunderkind college dropout drove most of the value creation.
In the modern era, Peter Gassner, founder of Veeva was 42. Diane Greene founded VMWare at 43. Brian Acton was 38. You start falling into the 30s fairly quickly.
And if we look in terms of value creation, the crazy outsize wins come from founder founders. E.g. the GAFA companies were all founded by sub-30s.
This study, like most of the Kauffman Foundation studies, tend to rely on obscure methods to reach these counterintuitive results. It's not to say 45-year-olds can't build great companies, but if you made a list of the 150-200 most notable tech startups of the last few decades, you'd see an average age closer to 33 than 45, even lower if you don't include the 2nd or 3rd startups of some founders.