Some thoughts...
Entrepreneurship seems slightly misstated here and some conclusions are suspect, and this is why:
Employers are looking to remain flexible, so they're hiring independent contractors, which means that one has no choice but to become a sole proprietorship, which is the automatic designation when one becomes self-employed.
To confirm this point, the study specifically states that the data it takes from the CPS includes, "all business owners, including those who own incorporated or unincorporated businesses."
Secondly, looking at their conclusion of age group changes relative to the number of the self-employed, it's also possible that the rise in entrepreneurship (forced self-employment) among older Americans reflects age bias in hiring. There is some anecdotal evidence that this is occurring, at a time when employers have greater room to be picky (big labor supply, low labor demand).
