It looks like MOST of the descriptions people are using for "richness" are pretty vague, which I find to be weird since we actually have readily available numbers.

Median US individual income is $26,695 . Unless you want to claim that half of Americans are "poor" or lower class, you should probably start your middle class no lower than that.

(ETA: For a full-time worker over the age of 25 it's $39k, so you could maybe push it up to that, but it disregards a lot of people who are stuck in part time jobs, or are kept working at just below full time so that they don't have to be given benefits)

10%ers start at $82k. I think it would be silly to say someone in the top 10% of US earners isn't rich. Almost all STEM LWers I've met either make well above this, or work for a non-profit.