Users get filtered results for the sensitive queries and a message asks users to prove they have more than 19 years to see the unfiltered results. According to Google Translate, an approximate translation of the message is: "Information networks and information protection, such as promoting the use of the laws and regulations by the Youth Protection Act under the age of 19 except for the result has been harmful to the youth. All users are over the age of 19, you can see the results."
To verify the age, users have to enter the name and the social security number. Apparently, this information is not saved on Google's servers and the result of this verification can be saved to a Google account.
Most other sites that operate in South Korea use age verification, but it's not clear if Google can create a connection between the identity of a user and his future queries. According to Wikipedia, "South Korea is in ONI's substantial category but is not on RSF's internet enemy list. South Korea has banned at least 31 sites considered sympathetic to North Korea through the use of IP blocking. In 2007, numerous bloggers were censored and their posts deleted by police for expressing criticism of, or even support for, presidential candidates."
{ via Flickr }