My extended family and most of their neighbors are Costco shoppers. So, while I cannot point to statistical studies, I can speak from first-hand "annecdotal" experience.

Yes, from what I've seen, Costco shoppers tend to range from middle class all the way up to the very wealthy, believe it or not, depending on region and culture.

The region I'd point to in this case is the state of Utah, including both LDS and non-LDS individuals. Households would often be affluent. Professions include doctors, dentists, senior engineers, business owners, heads of hospitals. Education would usually be a bachelor's degree but including many MDs and PhDs.

Often the women do the shopping, but some of the female Costco shoppers I know personally have advanced degrees and high paying full-time jobs. Men also shop at Costco -- especially when preparing for large events that feed multiple families. Typically Costco is not the only place people shop -- but the most time-efficient. Cost is actually not the primary motivation, from what I've seen. Time is. People are seeking a single, simpler bulk solution.

"Families" is the most important word here, I suspect, when defining the Costco demographic. At one point years ago, I inherited a Costco member card; but I found it unwieldy to shop for just 1-3 people there. Generally, from what I've seen, Costco shoppers represent families with at least 2 children over the age of 8.

For what it's worth, that's my unscientific answer.