Correct; there are no state or federal laws requiring films to be submitted to the MPAA rating system, or requiring theaters to enforce said ratings. It’s perfectly legal for any move theater in the country to let unaccompanied minors into an R rated or even NC-17 move (not that latter rating get’s used a lot).
The first R movie I saw with a parent was One Flew Over the Cuckoo’s Next. That one was probably correctly rated for adult themes, as much for the naked man’s butt. My mother had to explain lobotomies to me, and even told me about JFK’s sister having had one.
The first R film I saw by myself was The First Monday in October, about a woman on the SCOTUS (made several year before the O’Connor appointment). I expected to see or hear things I’d never seen or heard before. It didn’t seem any different from PG films I’d seen, other than most films I watched didn’t have so much political humor. I was 14 and just walked in, but at 14, I looked a little ambiguously older. I mean, if you saw me with other 14-year-olds, I blended in, but I had a 34-inch bust, I was 5’3, and I spent lots of time with college students and adults, so I didn’t have typical high school freshman speech patterns.
I saw Victor/Victoria with my cousin and some friends a couple of years later. That film did briefly have naked breasts in it, but I think it mainly was an R film in 1983 for showing men in bed together-- not doing anything, but with the implication that they had or would. I wonder if it would be an R film now. It’s one of the most matter-of-factly positive gay films there is. I mean, it’s not really a gay film, like Jeffrey, or Maurice, or Desert Hearts, but it’s so gay-positive.
They were checking IDs when I was a teen in the 60s and 70s. You couldn’t get in R rated movies if you weren’t 17 or older unless you were with someone who could pass for your parent. A lot of those movies aren’t rated R anymore.
It’s a fantastic film, one I saw multiple times in the theater. But Victor/Victoria was PG. There’s brief semi-nudity in a couple of the nightclub scenes, though it’s “artistic,” and while there’s a lot of talk about sex, and people are shown in bed in various pairings, there’s really nothing that would earn a higher rating. Maybe if PG-13 had been around then, it would’ve been a contender, but definitely not an R.
My aunt once babysat me and my sister when we were… probably 10 and 12, I think? She had some errands to run, so she dropped us off at a theater where I can only guess she thought a kid’s movie was playing. But while Duchess and the Dirtwater Fox may sound like a Disney film about a roguish cartoon animal (maybe a follow-up to Robin Hood!), it was actually a raunchy comedy set in the Wild West about a showgirl/hooker who teams up with a con artist/card sharp to steal a massive amount of moolah while avoiding capture from all the people they’ve cheated. It’s actually pretty funny, in a sub-Mel Brooks sorta way, but not a kids’ film. Still, it was PG.