Originally Posted by Wikipedia, on Parental Advisory
Some retailers (such as Wal-Mart) refuse to sell albums containing the label, and many others limit the sale of such albums to adults only, although, most stores have settled on an age limit of 17 in order to buy an album containing the label. In some countries, however, such as the United Kingdom, albums displaying the sticker are available for purchase by persons of any age. While the label is mostly prevalent on rap and rock albums, it can appear on any genre of CD which the RIAA believes warrants the need for one.