I think it's because young people try to solve (most of the times) young people's problems (music, dating, photos, food delivery, code and technology tools, movies, social activity, even taxi riding, most people with kids buy a van and move to the suburbs). Young people also tend to have a slightly bit more free time, and are more likely to share things they use and like. And I don't mean only teens, I also include professional, smart, rich, educated 25 years old who work in a startup, or own a company, or sold one, or had stocks in one and now are millionaires, and even become VCs, mentoring and helping other startups.
There are billions of dollars on the table that "Old people" companies are taking. Look at all the boring enterprise Java jobs out there, these people are selling stuff. to other companies you haven't heard of, solving business problems you haven't encountered and perhaps never will if you are a 25 years old hacker, because you will never want to go work for these companies.
Older people work there, and even if they have a great idea for a startup they are much less likely to do it.
1) health insurance - prime reason. older people tend to have kids, or in the making. Unless they have some savings, they can't risk losing their health insurance.
2) family, yes, there are successful founders with kids, but it is much easier to start a company when you don't have any. how many people with families move to SF for 3 months for YC and leave their kids and spouse at hime?
3) age bias. VCs look at older people like this - if you had what it takes, why did you get a boring Java job for 10 years till you decided to jump into the water?
4) Enterprise sales is hard. it takes sometimes 2 years of a sales cycle to close a deal. but when you do it can be a 7 figure deal easily.
5) What the article said about young tech writers, I agree