I don't think there's a minimum. There's plenty of startups that were started by college students or even younger. As long as you're smart and know where you want to take your startup, you can succeed.
I was wondering how many years one should put in at a company right after school, in order to have an experience safety net should they want to work at a traditional company once more, if their startup fails.
[1] http://www.nytimes.com/2013/07/20/business/recent-graduates-...