Looking at child pornography is not a crime. Knowingly possessing child pornography is a crime. Just because someone is labeled a "teen" does not mean that they are under the age of 18. In order to be convicted of possessing child pornography, you have to knowingly possess pornography involving a person whom you know to be under 18.
By viewing child pornography, that person would be in possession of it. Just because it's sitting on a screen instead of downloaded, they are still potentially in possession.
I'm not sure what your question is - whether it's a priority for law enforcement to crack down on websites that have questionable images? I'm sure law enforcement would care and do what they could to crack down on the site.
Are you asking about somebody who looks at pornography sites that has child erotica or legal aged girls that appear younger? Those sites are legal.
Both state and federal case law say that merely viewing child pornography is not possession. See, for example, Tecklenburg v. Appellate Div. of Superior Court (2009) 169 Cal.App.4th 1402, 1416-1419.