So common that by the time I joined the Marine Corps at 17 years of age, after years of gang showers in high school, I had never seen an uncircumcised male. Not once!
We had a pair of twin brothers from Canada in my boot camp platoon, and when I saw their hanging foreskins in the showers, I honest to God had no idea what I was looking at. I thought they had some sort of shared birth defect!
A few years later when I was stationed in West Berlin and exploring the German gay scene, it took me a while to adapt to the fact that practically every man I met at the bars was uncircumcised.
Circumcision isn't quite that universal in the United States today, that a teenage boy would be shocked to see a foreskin, but it's still pretty ordinary.