But not controlling for age is still a major omission. A lot of middle class people are in the top 10% as they near retirement because they've been saving throughout their life. How many? The article doesn't say.
If you restrict to 65-69 age group, the median only shifts to about 270k, 90th percentile is a little under 2mm.
To get to 1mm net worth at that age group, you have to go up to about the 80 percentile.
It seems you are correct that the 90 percentile shifts a fair bit at retirement age (65-69 is peak net worth) but it doesn't effect the middle very much, and you have to be > 20% to catch that 1.25mm number at any age group.
If you wanted age normalized wealth, the top 10% would probably be a lot of rich kids whose peers have negative net worth. I'm sure that's an interesting cut, but it's a different topic.
The wealthiest do tend to be older because they've accrued wealth over a lifetime, but it's definitely not true that if you're old, you're wealthy.
Wealth is not income, and neither are class.