[ghaff]

>easily retire in their 40s

That's a stretch. An average of $150K/year before taxes throughout 20s and 30s is a pretty good job in the US. Say they save $50K/year--which is a lot on that salary--that's $1million saved overall which, depending on your assumptions, will give you about median US household income annually. So possible in a sense if retiring as soon as possible is your goal but certainly not to everyone's tastes.

[jmstriegel]

I think this doesn't account for investment returns. If you assume this money is invested for the duration of your 20s and 30s, you "only" need to invest $50k/yr for the first 10 years. At an average 7% growth, you can let it ride for a decade and still have just under 1.3mm by age 40.

In your example, a high salary individual contributing $50k/yr for 20 years at 7% ends up with over $2mm by age 40. That's $80k/yr at a %4 withdrawal rate for the rest of your life.

More reasonably, a $25k/yr contribution for 20 years at %7, would pass $1mm by 40. If you let that sit for the next decade and retire just before you turn 50, that will roughly double over the decade to $2mm.

I agree that this isn't attainable for everyone, but contributing the $19.5k/yr max to a 401k pre-tax, and $5.5k/yr into a Roth IRA over your 20s and 30s, will likely make you a millionaire by 40 and a 2-millionaire by 50.