They are only worn after you are admitted to the temple, which is usually when you are married or when you are preparing to leave on a mission.

From my experience, your average Mormon would totally wear garments. All the average Mormon kids I grew up with went to seminary before school every morning, and many of them went on to either serve missions or were married in the temple. And not all of those kids dressed modestly Any Mormon who is bothering to follow modesty dress is certainly wearing garments if they are old enough to have received their endowments in the temple.

It's not Salt Lake City, but just about every inland area of California is pretty much teeming with Mormons.