"HELLO SEXY," says the message. "MY NAME IS HEIDI AND ME AND MY FRIEND ARE HORNY. CLICK HERE TO CUM JOIN THE ORGY PARTY NOW". Welcome to the world of my Hotmail inbox. This is spam: every time I'm logged on, friends of Heidi visit like persistent - and, I should point out, uninvited - door-to-door salesmen.

But then, it's never been hard to find porn on the net. For those on the lookout the stuff is everywhere, not just on websites but in newsgroups and chat rooms - and, according to a new survey, those on the lookout increasingly include children and teenagers.

The research, from net monitoring agency NetValue (you can read it at uk.netvalue.com), found that one in five online under-17s use porn sites - remaining for an average of 28 minutes. On one level it's surprising that the number isn't higher. The survey also found that more than one in nine children under 17 connect to the net at home - and that the UK is second only to Norway in the numbers of young users. The number of under-17s online in the UK has grown by 44% over the past six months, from more than 1m in October to almost 1.5m in March.

Under-11s account for 249,000 users in the UK, with 1,236,000 users aged between 11 and 16. There is web-based software that is now available to stop children accessing unsavoury sites - which can be so stringent that news sites often find themselves closed off. But any imaginative teenager can get round those restrictions in 10 minutes and access what they like. According to the survey, more than one in four also visit gambling sites, which shows exactly how ineffective age restrictions on websites are.

Just as we wouldn't have the video industry today without porn, we wouldn't have commercial websites either - almost all web e-commerce models were originally tried out on adult sites. A friend working at a web broadcaster tells how all the video editors would scour adult sites for hints on improving their live streaming. "Look at the pixellation on that," would be the cry. And many, many net operations were built, at least partly, on porn. Take Yahoo! for instance, which has caused a minor revolution among some of its users by withdrawing adult material from its sites. Go to Yahoo! UK now, type the word "porn" into a search engine and it will bring up a worthy page titled "pornography issues" and an interesting article in Salon magazine about Louis Theroux - but not much else. It's a far cry from the old days of Yahoo!, which has sold X-rated products for two years and even employed staff who specifically surfed adult sites for the directories and placed sex banner advertising on connected searches.

Now that is officially over, after the Los Angeles Times ran a piece criticising a decision by the portal apparently upping the amount of porn content. Following the French court case over the auction of Nazi memorabilia on the net, the last thing it needed was another outcry. So Yahoo said it would cease selling adult products, and accepting banner ads from porn sites, and auctioning adult movies, and would cut down on sex-related message boards and chat rooms. It's not alone: Lycos is reassessing its porn policy and MSN has reaffirmed its blanket ban. Big search engines like Google and AltaVista still return adult content without question but the general move is towards a more Disney-fied net.

But this fails to deal with the root cause. In the same way that a video and TV in the bedroom can be used as a substitute for childcare, surely any parent who provides unlimited access to the net for a young teenager is more responsible than the portals and gateways for what those teenagers access? At least the video market is regulated in a consistent way. How much worse is the potential of the net, where regulation is an Orwellian nightmare. And there is no system yet that would work. For a start, it would have to be international and run by a body with clout - can you imagine some official at the UN publishing approval ratings for a website?

This is the question that they face: mainstream portals blocking access to porn won't stop it existing or prevent the insatiable demand for it. But what it could do is take away the essential principle of the net: the free movement of information. And, sooner or later, someone will have to deal with that.

There is one issue missing from this debate, of course. It's that now old fashioned word: "exploitation". While middle-class parents worry about what their child is up to in the bedroom, there is another group forgotten by them and, I suspect, by web users too. This group is largely silent, witness only to the whims of someone with a webcam or a digital camera. These are the children of the same age who feature in the same websites. They could do with some concern, too.

• Simon Rogers is deputy editor of Guardian Unlimited