plenty of companies got started more than 20 years ago. What would have prevented you from starting a company when MS or Apple started?

Having a grandfather worth $20 million.

Seriously, don't kid yourself. It was a VERY DIFFERENT world back then. You graduated college and hoped you could find a job. There was no www and you needed 5 to 10 grand just for hardware and proprietary software. It's utterly stunning how easy it is to get started today compared to then.

(Bill Gates and Paul Allen went to the private Lakeside School which had a privately donated PDP-11 in 1968. Imagine being 13 years old and having access to the same computing power as Stanford University. The other 99.99% of us had to wait another 10 years for access to a computer.)

Software was a lot harder to write when I was in high school.

Also, entrepreneurship wasn't encouraged.

Which means there was a lot less competition.

Mindset and almost no open source + internet/Arpanet infrastructure. (Internet changes marketing and distribution to the advantage of young entrepreneurs.) Twenty years ago, kids didn't have a long list of successful role models who made it big while young. Now, that's all you see in the media.

Lack of physical existence for one.