Strangely, the Parental Advisory sticker is not legally binding at all. It was invented by Tipper Gore in some sort of attempt to stop parents letting their kids listen to rap music. It completely backfired, since the Parental Advisory sticker is now an advert for how cool an album is, and is frequently used as a marketing tool. It even looks pretty cool, and fits well on any album cover since it's black and white.
The thing is, there's no law about who can buy albums with Parental Advisory stickers on them, which rendered them totally useless in the first place. Sure, shops can refuse to sell anything to any customer, but few actually do. Most clerks won't even look at what it is you're buying, unless it's a video or dvd, in which case they have to check for the certificate if there is one. Sure, there are some clerks who enforce their moral code, and some outlets which have a policy of not selling these records to children, and that's their right, but it has nothing to do with the law. In fact, it's rare for that to happen because there's always someone who will supply kids with the record, or someone who will buy it for them, so it's not good for business.
Recently, the mainstream press have been crying out about how easy it is for kids to get hold of explicit music, even though they've been doing so for years. This is no doubt due to nu-metal and Eminem becoming so popular. Consumer programs like Watchdog were shocked to learn that a 12-year-old girl could easily buy albums by Eminem and Limp Bizkit (I was shocked too, but for different reasons...) that had Parental Advisory stickers on them. Ironically, this is something that most people who listen to rock or rap music have known since they were 12.
Watchdog asked if something might be done to stop children buying these records. There was talk of applying an age limit to records with Parental Advisory stickers, but the age limit was not specified and to be honest, the politician in question was probably just trying to shut them up until they found something else to start a moral panic about. Besides, even if an age limit were applied, it wouldn't do much good, since the real punchline of the parental advisory sticker is that it's voluntary. In fact some albums that contain the most outrageous lyrics ever don't have one. If an age limit were applied, record companies would merely stop putting the stickers on, and probably find some other means of showing how much this album will annoy your parents. And no, nobody's going to censor records like they do films, because that would take up far too much time.