There is no set path for these things, but if there was an ideal path it would probably be working as an early employee < 15-20 at a startup. While at the Startup you get real-life experience working at a company, you meet new people and mature. Maybe you stay 1,2, or 3 years, then you go off and do something on your own. Meeting/Networking is crucial because your co-founders have to be good matches.

Grad school would be great if there is a purpose. ITP would be good if you want to be a founder that leans product/design. Stanford/MIT would be good if you want to lean more in technical.

Y-combinator is the startup-MBA. It is an amazing program that I would recommend to any entrepreneur at any stage (college undergrad, grad, 1-10 year, or older). My issue with someone from college jumping right into YC is lack of life experience and technical experience. There are exceptions to that rule, and PG does a great job in picking those young entrepreneurs that can handle it. So if you feel you have what it takes, apply.