I think this points to how much easier it has become to start a business nowadays (a normal business, not a tech business). I'm sure some of this is due to the marketing revolution that is the web, but I think it's also due to the move to services that has happened in the USA over the past 4 decades. (It's a lot easier to run a one person services business than a one person manufacturing business.)

Pair that with a) boomers needing more money because they have fewer pensions and are living longer, b) older folks have some accumulated capital (either financial, human or relationship) and c) the flexibility of a lifestyle business (even if you work longer hours, you can timeshift), and it doesn't surprise me that older folks are starting more businesses.