One thing I've noted in general is Costco employees are much younger on average than Walmart employees.

I almost never find an old employee at Costco, while at Walmart most of them are old or middle aged.

Walmart often has stores in poorer areas than Costco, which is likely why you see the age discrepancy.

Costco also has a pretty robust internal promotion system from what I've heard, so older employees might also be occupying management spots.

Though it might also be that nearly every piece of stock at a Costco will be significantly heavier than that at Walmart, making it difficult for older people who might have more in the way of back problems etc to work there.

Not really, my experience is based on both in the Bay Area, and includes WalMart neighborhood stores.

I think it has to do with lot of older people shopping at Walmart so they feel comfortable around others. Don't know.

your experience is literally in the most HCOL city in the United States.

This is my observation too, I often wondered if Walmart wasn't around what would be these folks alternatives?