Those who are under 30 and make $300k+ a year, what do you do?

Typically for people under 30 making that much it's going to be a broad range of business owners/successful entrepreneurs that got started young, finance jobs, and some technology jobs. It's less likely to be healthcare related as most physicians will not complete their residency training until their 30s (and even then will likely not be making the amount in question). Some dentists do finish their training before their 30s and can obtain a high salary, although they may have to take on significant debt opening or joining a practice. Also add in successful professional athletes.