You are thinking here, ratings in Japan are different. Video game ratings there are A, B, C, D, and Z. It's much easier in other regions (ESRB has the age name right on the rating, like "Everyone" and "Teen"; PEGI uses the minimum age, so ratings like 3+ and 12+). Japanese movie ratings are more clear: "G", "PG-12", "R-15", "R-18".
As for R-Rated movies here in America, I won't even get into how ridiculous it is that some politicians want to make it illegal for stores to sell M rated games to children (something that thankfully every court so far has ruled to be unconstitutional) but they don't want to make it illegal to sell R rated movies to children.