Obscenity and age of consent laws are different restrictions. AOC laws focus on the age of the person whereas obscenity focuses on the acts of the person. A video can feature people over the age of consent and still be hit by obscenity charges because the acts depicted fall into the narrow definition of 'obscene' laid out in the MILLER case.
http://caselaw.lp.findlaw.com/script...l=413&invol=15