In my opinion, the US rating system is insane, because it neither treats adult audiences like adults capable of choosing their own entertainment within the law (R rated films getting cut, mainly due to sex - see for instance Possessor and Infinity Pool, which were fully uncut here), nor does it properly protect children (small children being allowed to attend R-rated films if accompanied by adults). I once witnessed a child who can't have been older than about three or four alternating between terrified and bored in the R-rated Fallen (1998). I wished they'd taken said child to see The Jungle Book (1967), which was playing on rerelease in the next screen.

Here in the UK, we've pretty much won the war against the film censors (it was hard won, believe me, and I was personally engaged in fighting their classism and hypocrisy as a student). Now, we have U, PG, and 12A (very roughly equivalent to G, PG, and PG-13 in the US), then 15 and 18, which are mandatory minimum age admissions. These are typically R-rated films or NC-17 rated films, with a few rare exceptions (The Batman was considered strong enough for a 15 rating here).