There have been various film rating/certification systems over the years in the U.S. and other countries. TMDB only defines codes for the current ratings. This leaves a ton of older films with certifications which can not be represented by TMDB.
Solution: Add additional certification (tags) to represent the union of all certifications. If an ordering is required, then one can be cobbled together. For the U.S. I suggest adding the tags: "Approved" (Hays) (I have also seen "Passed" used) "M" (mature) "SMA" (Suggested for Mature Audiences, but rarely used) "GP" (more or less replaced "M") "PG-13" (more or less replaced "M") "X" (more or less replaced by "NC-17").
A pretty good summary of the ratings used in the US over time can be found at: https://en.wikipedia.org/wiki/Motion_Picture_Association_of_America_film_rating_system. It does not address the earlier rating of "Approved" (or "Passed"), used to indicate that a film was compliant with the "Hays"(pre-1968 code). Note that in addition to Film ratings, there are TV ratings. I tend to ignore TV ratings, however there are some "made for TV" films which have no other ratings. At the moment I think that adding support for TV ratings to regular films should be addressed later, but not forgotten. Further, note that there can be different cuts of a film with different ratings. An example of different cuts of a film may include an X/NC-17 un-cut version, an R version (for theatrical or DVD). This is already handled by different releases. Presumably, a film with both a film and TV rating would have separate release entries for each.
I live in the world of old movies, all the way back to 1929 or so. But you only have to go back to the 60's before you start seeing a large number of "unrated" movies in TMDB because there is no support for the assigned ratings. I have seen some "Approved" movies being given a "G" rating in TMDB, but they are not the same.