I'm guessing none of these are in Vegas. I think I can count on one hand how many times I've seen someone get carded and kids run rampant in some of the casinos here.
It was the same for me when I went to Vegas in 2006.
I went to a handful of different casinos and drank like a fish, yet despite being barely 21, I only got asked for ID once over the course of 3 days.