Not all people who end up addicted to pornography are addicted immediately. But some can be (hence the reason to help educate children about the potential impact/drug-like effects of pornography and to keep open communication with them so they know they have a safe place to talk about hard things).
Mark Kastleman, author of The Drug of the New Millennium: The Science of How Internet Pornography Radically Alters the Human Brain and Body, shares a little about his first exposure to pornography and the effect it had on him.
More from Mark and others who have been affected by pornography addiction (either as the one with addiction or a loved one of someone with addiction) can be found here.
p.s. For parents whose children have been involved with pornography use, you might benefit from various videos by Jeff Ford about creating a safe space to talk about hard things with your children.
See the following links, called his Five Cs for talking to children about hard/dangerous things:
Part 1: Compliment http://www.youtube.com/watch?v=9K__dKK3mik&feature=relmfu
Part 2: Commit: http://www.youtube.com/watch?v=tvtiCzRnly0&feature=relmfu
Part 3: Calm: http://www.youtube.com/watch?v=qhp2mEL7MUE&feature=relmfu
Part 4: Composed: http://youtu.be/q3vRZuqXIfk
Part 5: Connect: http://www.youtube.com/watch?v=_7DX8KBMws4&feature=relmfu