Interesting information.
I remember when the PG-13 rating was introduced. It was a pretty good idea, a way of indicating that a movie had a bit too much sex and/or violence for a PG rating, but not so much as to deserve an R rating.
The only time I have EVER seen anyone get carded for an R-rated movie was when I went to see the South Park movie. I wasn't carded, but some of my friends were (we were all in our early to mid 20's). Meantime there were a bunch of little kids (4-5yo) in the theatre with their parents. I guess their parents thought it was just a cute cartoon, never mind the R rating!
When I was 16, a bunch of us went to see Universal Soldier. Most of us were 15-16 and could be taken for 17, but there was a 12yo boy with us, who in no way could be mistaken for 17. Were we carded? Nope...