If the media is to be believed, most of that 'indecency' falls under sexuality. Teenagers are probably the ones that are the most interested in sex. They're naturally curious about it. Society has determined for them that they should be 'protected' from access to sexual information on the net. At a national and planetary level it's an impossible task, even if it were a desireable one.
Technological advances go against sexual taboos.
If 12 year olds want to know about sex they should be able to get information about sex , if they're not interested they won't look for it. One must look for pornography on the net, it's not like it comes to your mailboxes. Besides decency is subjective. But since decency is subjective it falls on parents to make that determination on their own rather than look to government to do it for them. The net, being what it is, has provided ways for parents to do just that through screening software, without restricting the rights of others.