It's never reasonable to "go bare" in the US, no matter how young and/or healthy you are. You never know when your gall bladder or appendix will decide to turn on you, or when a hernia will pop up (or pop out, as it were), or when an unexpected pregnancy will occur- to say nothing of bike accidents or other misc. injuries. One day you're feeling fine, the next day you're in the hospital with a ruptured appendix and end up with a $20,000 bill. Oh, you had a bad reaction to the anesthetic they used, and needed an extra day in the hospital? Make that a $25,000 bill. Thank you, come again!

Also, besides the immediate proximal financial impact of having to deal with any of those situations while uninsured, remember the pre-existing condition trap: while you're in the hospital getting that appendix out, you'd darn well better hope that your CT scan doesn't turn up anything else of interest (cancers, hernias, etc.), as anything they discover while you're uninsured will be most likely be excluded from whatever future health coverage you might end up with. Oh, wait-- that'll probably be a moot point, since insurance companies usually refuse to sell individual policies to people with pre-existing conditions (even to otherwise-healthy young people with minor conditions).

Catastrophic coverage is an absolute bare minimum to have, and, even then, make sure you really read the fine print. A lot of catastrophic or other low-premium plans have pretty major strings attached in terms of what they will and won't cover.

Another important point: simply by virtue of having insurance of some description, you will get different pricing than if you're billed directly as an individual. That's because insurers pay the claim and then settle it with the policyholder, so the healthcare providers don't factor in the same delinquency / nonpayment risk that they apply to people who just walk in off the street and presumably pay in cash. You may get invoiced 2-2.5x more for the same procedure than you would if the billing were going through some kind of policy, which is in itself significant, never mind the limits, deductible or possibility of rescission.

In regards to young people, that only works if they have no preexisting conditions. Otherwise loss of group coverage risks long term loss of coverage.

You can transition "pay yourself" from your employer's (or parents) plan without running into the pre-existing condition problem. Preexisting conditions are only a problem if you have a gap in coverage.

Besides, most people don't have preexisting conditions, so the ones that do are treatable as a special case.

Through an employer, premiums are paid with pre-tax dollars whereas on your own, they are paid with after-tax dollars.

Pre-tax vs post-tax is only relevant if you have taxable income, which isn't the case for "quitting work to do a startup".

I've got a suspicion that few of you have actually run the numbers, so I'll help.

I'm over 50, male, unmarried, and in San Jose and my reasonably comprehensive plan cost $325/month.

If you're 20-30, you could get the same for significantly less, and you'd be adequately served by a somewhat lesser plan. (I'd have a lesser plan if I was younger.) If you're somewhere else (not SF), or are married or female, you can do better.

Note that $325/month is significantly less than rent.

Health insurance actually isn't a significant obstacle for the vast majority.