I think a lot of the reasons can be found in that answer.
1. Bill Gates
2. Larry Ellison
3. Jeff Bezos
4. Sergey Brin
5. Larry Page
6. Steve Ballmer
7. Paul Allen
8. Michael Dell
9. Laurenne Powell Jobs (widow of Steve Jobs)
10. Mark Zuckerberg
If the definition of old is "younger than 20 years", this represents 3 startups (Amazon, Google, Facebook). If the definition is "younger than Skype", then only Facebook is left.
I was more interested if younger successful founders in the US faster take-over older founders. And they don't. A large part of the wealth of the older founders has been acquired past-IPO.
The conclusion (for me) is that to make it into the top 10 (whether US or Europe) and stay there, you gotta grow a company successfully for many years, like Microsoft, Oracle or Apple did in the US and SAP did in Europe.
Discounting the Samwers for obvious reasons.
We suck compared to the US regardless of how you slice billionaires lists.
I find it interesting they have a very entrepreneurial YAGNI and fake-it-till-you-make-it swagger. They really do a lot with very few resources.
Also, I'm not sure what Alan Turing has to do with Sinclair and Amstrad failing and Dell being successful. From what I understand of the early development of the computer, much more work was done in Boston, NY, and the Valley than in the UK.
But pg's point was specifically about how the Euro top ten was dominated by old companies and boring business models. My point is that aside from google, amazon and Facebook, the US top ten is not all that hip and fresh either.
That's a pretty big aside.