18+ vs R-rated

18+

adj
  • Suitable only for adults; mature. 

R-rated

adj
  • Not suitable for children, especially due to involving excessive sexual, violent, or rude content 

  • Of a motion picture, having an "R rating" from one more motion picture rating authorities, typically meaning it is suitable only for adults. 

  • Of a motion picture, having an "R rating" from the Motion Picture Association (MPA), meaning it is suitable only for adults. 

How often have the words 18+ and R-rated occurred in a corpus of books? (source: Google Ngram Viewer )