In a paper entitled Internet Filtering and Adolescent Exposure to Online Sexual Material, Oxford Internet Institute researchers Victoria Nash and Andrew Przybylski found that Internet filters rarely work to keep adolescents away from online porn.

 

“It’s important to consider the efficacy of Internet filtering,” said Dr, Nash. “Internet filtering tools are expensive to develop and maintain, and can easily ‘underblock’ due to the constant development of new ways of sharing content. Additionally, there are concerns about human rights violations – filtering can lead to ‘overblocking’, where young people are not able to access legitimate health and relationship information.”

Source: Researchers find that filters don’t prevent porn | TechCrunch

The article is talking about home use, where in the EU and UK approximately 50% of households have some form of internet filter installed.