Isn't it all about experience - most of those companies were starting from scratch in newish industries where no one had experience. But most companies are doing something where there are other companies doing similar things. For example most successful bio/chem/fin tech companies were formed by people with experience in other companies who understand the tech and market - some younger and many older.

Nowadays if you were looking to fund someones idea for a social media company you might prefer someone who had worked in a senior position in another - perhaps 29 or older rather than 19.