There are certain cases where porn can be harmful even to adults. In one of my second year psych classes we read about a guy who watched copius amounts of pornography, and then kidinapped women and did the things he saw on the pornos to them. After he was sentenced he tried to sue the porn companies for 'ensnaring him'
This is a extreme case but it can show us that anything can be dangerous when goes overboard. It also shows us that pornography can be addictive, jsut like all of the other good things in life so as Eric said, moderation is needed.
As for children, I believe that around the age 12 and onwards is OK to be viewing pornography, so long as it is not on a regular basis and adults have already explained to the child why those two (or more) people are having sex in front of the camera and can put it into context for them so they dont go around thinking all hot girls/guys are sex objects.