I'm a huge anti-censorship freak, as you all know, but I also believe in parents' rights, and I think it's reasonable that buying / selling / exchanging certain "mature" products or services (sex, drugs, alcohol, violent stuff, etc) require explicit parental permission. Sure, it would be nice if the parents could be expected to monitor their kids 100% of the time, and emerging technology is gradually making it easier, but it still seems very far from realistic.
In an Anarcho-Capitalist society, responsibility for "
unconsentual" transactions in theory could be enforced through litigation - if you sell my kid something that can cause mental or physical damage, I should be able to sue you. This would of course have negative side-effects, one of those being that even selling eggs and peanut butter to anyone who isn't an emancipated adult carries a potential liability. What if the kid eggs someone's house, or what if s\he's allergic to peanut butter? On the other hand it would be fairly easy to establish a system of relaying parental consent: malls may require under-18 shoppers to use a special credit card that they can only get with parents' permission, etc. Most teenagers would have this explicit permission to buy the things that teenagers are typically allowed to buy.
A bigger problem is the issue of less formal exchanges of information: can I sue you for just talking to my kid, or seeding a bittorrent file that my kid downloads?