You're including non-adults (330m population @ 6%). It's closer to 8% of the adult US population. Not very many people under 18 are millionaires or reasonably likely to be. The non-adults in question fall under the millionaire household grouping.

And if anyone is interested in the household wealth distribution (as of 1Q21):

Top 1%: $41.5 trillion | 90%-99%: $48.8 trillion | 50-90%: $36.5 trillion | bottom 50%: $2.6 trillion

$129.5 trillion total household assets.

There are around 128 million households in the US. The average American household is now worth a million dollars (the median is obviously far lower).

That top 1% of US households - 1.28 million households - holds an average of roughly $32 million in assets.