I don't see the math to retire at 30 without a crypto jackpot or inheritance.

The math is quite simple. Essentially the two variables we need to know are when our hypothetical FIREee starts working, and what lifestyle they are willing to accept in early retirement.

-Now Jacob at ERE probably sets the low end of the distribution of the early retirement with an initial budget of only $7,000/year. But let's be generous and multiply that by 2x to $14,000/year.

-If a person first earns a college degree,* they might enter the workforce on their 22 birthday. As the Shockingly Simple Math post shows, if they save 70% of their post-tax income, they will be FI in 8.5 years, halfway through their 30th year.

-If their spending rate is $14,000/year, then a savings rate of 70% requires (14,000/(1.00-0.700)) = $46,700/year in income,** which is no unheard of for a college graduate with an highly employable major.

Now you or I can certainly argue with the degree of safety against unexpected future expenses provided by an annual spend of only $14,000/year. We can also state that $14,000/year is an annual spend so low that you or I would be unhappy. But the key thing to remember here is that individual people's tolerance for risk and breakeven points where they cease to receive increases in happiness are different. And the two criticisms described earlier in this paragraph are the exact same criticisms people on bogleheads tend to have when MMMers venture over there and mention planning to retire with less than $3-5M saved.

*If you started straight out of high school at age 18, you would need a 60% savings rate (12.5 years to FIRE), and thus a take home income of (14,000/(1.00-0.600)) = $35,000/year. As this works out to $17.50/hour pretax it is, indeed, a rather high target for an 18 year old with a high school diploma.

**Actually probably a bit more to account for payroll taxes and federal/state income taxes, although hopefully they have access to a 401k + IRA and are able to minimize the bite of the latter.