User blog comment:Argorrath/Activision confirms next Call of Duty for this year/@comment-121.218.238.54-20130210040226/@comment-1187559-20130212232445

So you know the age rating just applies to people buying them, not whom plays them. Ergo most underage players simply have their parents, older relatives or whatever buy the game for them. Just like 18 rated movies, you can stop someone buying a ticket at the movies, but you can't stop them watching it in their home.