I guess it depends what you call "significant". I am 40 and have over 200k in my 401k, which I think is significant. And I could most likely expect to live 25 more years. If there's a crash tomorrow, my money wouldn't grow the way I am hoping it will over that time, but I should come out ok considering that I will be getting discount stocks while the market recovers.

It is significant if you remain healthy and employed with income.

But it is basically nothing if you get laid off at age 56, and you can't find another job due to age discrimination, your COBRA runs out after 18 months, but you are not 65 years old yet for Medicare . Obamacare may be completely neutered by then, so private health insurance may cost $30k/year for a 57 year-old. You still have a mortgage, you can't afford health insurance, so you take a risk and decide to skip it, because you are healthy. Then you get pancreatic cancer, and without health insurance, your chemotherapy completely depletes your 401k in one year. Then you die of cancer at age 59, because you cannot pay for the treatments anymore.