I agree with the advice to get a normal-ish but high-paying job, save and invest a lot for around 10 years, then "retire" and think about doing a startup using the skills you have developed.

The craziest thing I would do is a) put my money in an index fund instead of some sort of retirement account, forgoing the tax benefits of a retirement account. And b) keeping a rather short runway.

If you're in the U.S., you can put your money in an index fund within a retirement account with potential tax benefits without any downside. Just open a Roth IRA at Vanguard, for example, and invest up to $5,500 per year in after-tax dollars. You can withdraw your previous contributions (but not earnings) at any time, tax-free and penalty-free. After maxing that out (and your 401(k) if available), you may be interested in the soon-to-be-released Schwab Intelligent Portfolios for additional investing.