You're assuming "retirement" is still at 65, but increasingly it isn't. The median is actually highest at the oldest age group on that chart.

Everybody also forgets to include the net present value of social security (basically an annuity you were forced to buy), which is a disproportionately large amount of the net worth for lower income people both because they don't have as many other assets and because the income cap limits how much it adds to the net worth of the people at the top. (Though people would generally have a higher net worth without it; it pays back less than you'd have from investing the same money in an index fund.)

The numbers don't change much at all for 80+.

Not to mention lots of people won't live until 80 much less 60.

And adding social security would increase the % of people who had 500k but would also increase the net present value of individuals @ the top 20%.