You want to buy an album with explicit lyrics but aren't 18?

Easy...take the sticker off and put it on the nearest NSync album. As long as the album isn't obviously inappropriate the cashier won't take a second look when ringing it up.

Of course this requires the sticker be outside the plastic wrap. Parental advisories aren't always done in such an easily avoidable manner..
Be creative. If the Advisory sticker is under the plastic wrap, stick the pricetag overtop of the advisory.
I haven't found anywhere yet that had the computer-lookup identify age-limited music, but then again I've been 18 for almost a year. And few cashiers care enough to ID in the first place..