Marketwatch had an usually long article touching on many of the reasons I've laid out about why retiring very early, like in your 30's, is a bad idea for most people. If the Marketwatch piece left out one thing that I think is a big part of the downside of the FIRE (Financial Independence Retire Early) movement, it's the inability for people to envision themselves at 50 or 55 or 60 and understand that you can still be very young at those ages. This creates a sense of urgency to retire now, what I think might be a false urgency. At 52 I feel very young. I am a more fit wildland firefighter now than I was when I started back when I was 36. I don't hike faster than when I was that age but my stamina on the fire ground is far better as a function of how my exercise habits have evolved combined with a better understanding, by virtue of experience, of the work required.
Being very blunt, someone who gets their financial shit together at a young age all the while taking good care of themselves can find themselves at 50 with a house that is paid for or just about paid for, big chunk of change in their 401k (more on that in a second) and, as a big point I've made before about this and that Marketwatch covers, be fairly close to having a complete 35 year earnings record for their Social Security benefit. At 50, you probably won't have 35 years but you could easily have 27 or 28 which would result in a much higher benefit than having 15 years of earned income. As quick reminder, the first step in calculating SS benefits is to get an average income based on 35 years of earned income. Someone with only 25 years of earned income will have 10 zeros go into their 35 year average. Most FIRE believers say they are not counting on SS and while they might truly believe that at 35 or 40, they might seriously regret that decision at 65.
The assumptions were pulled from an article by the Financial Samurai blog and seem aggressive in terms of how much people are able to put away but what if in working 25 years and contributing to your 401k you have $800,000 at 50 years old? That amount at age 50 could fairly easily work out to a sustainable, even if not lucrative retirement plan generating $32,000/yr based on the 4% rule. There's just one problem (at least one problem) which is having to pay taxes on the withdrawals as well as a 10% penalty for being under 59 1/2. There is a workaround for the penalty, which is to take substantially equal payments from your IRA under Rule 72t, presumably you've rolled your 401k over into an IRA. With $800,000 in an IRA, Bank Rate calculates you could take $34,349 per year until you're 59 1/2. If you only needed $25,000/yr then you could have two IRA's one with $585,000 in where the $25,000 would come from (per Bank Rate's calculation) and the remaining $215,000 in a separate IRA that you wouldn't be drawing from.
Of course this all presumes no accessible money outside your 401k/IRA rollover. Having $100,000 or $200,000 in a taxable account obviously gives much more optionality to this scenario as would having a home paid off at this age.
Some of the numbers I'm throwing around here might seem pretty big and based on various studies of average and median 401k balances (the second linked Marketwatch piece says the average balance was $100,000 in 2017 while the median was $27,000), the reality is that even retiring at 50 will require substantial savings.
A slightly different way to look at this, depending on how much money someone has at 50 or 55 and depending on how desperate they are to get out of the rat race, is to consider/guess how long they could make a smaller piece of money last before getting to Social Security. While I think this is a lousy way to go, I don't know what it is like to be in your 50's and despising your work but plenty of people are in that position. If you're 54 and have $540,000, can you make that last until you're 62? Can you stretch that sum out a little long to let your SS benefit grow, like until 64 or hopefully longer? Or in this scenario can you find something that maybe just covers the basics but gets you out of the rat race? Doing something you enjoy for half the money could pay the bills, maybe not allow you to save money but let your IRA continue to grow untouched.
There are a lot of variables and a lot of questions that people need to answer for themselves but the point I continue to make (despite getting torched for it at Seeking Alpha) is that you have far more optionality in the scenario of waiting until 50 or 55, you can still be very young at that age depending on your lifestyle habits and have many more years to enjoy being retired.