If ordinary company founders are not excluded, then distinguishing between startups and ordinary companies would not change the fact that most entrepreneurs are in their forties. If it's a distinction without a difference, why distinguish?

I'm not sure I understand you correctly, but he's saying that startup founders tend to be young, probably due to the risk profile of startups vs small businesses. Entrepreneurs as a whole (a much larger group) may well be in their forties on average, though.

Edit: Does anyone happen to know the success rate (for some definition of success) for restaurants?

According to this[1] Bloomberg article, failure rate is roughly 60%. Failure rate is defined as changing ownership within three years of founding.

Pretty low failure rate, compared to a startup.

[1] http://www.businessweek.com/stories/2007-04-16/the-restauran...

Funny that ownership change is usually considered a startup success.

You, are right; but there is ambiguity. 'Startup' is not never mentioned by the authors.