You are right, there is nothing to stop a 13yr old walking in and viewing a 18 rated movie but, at the same time, it shows due dilligence on the part of the producer by putting that rating on the movie.
Actually there is more protecting a movie from being seen by a 13 yr old, it is called the ticket taker at the movie theatre. They are the backstop to insure that the rating has effect.

Online there is no such person.

Also I'd say that perhaps we should be looking more towards television than the movie theatre as it is closer in approximating the problem online has in insuring who gains access or not.

Television today does have warnings throughout. After every commercial break a show repeats its adult warnings, whether it is about violence or nudity or whatever. The reason being that many access a television show after the beginning, which really is no different than an online website. People find a site not from just the index page, but from other pages that are listed either on Search Engines or link lists or wherever.

So any warning, to be effective has to be in place on more than just the index page, imho.

Software solutions have to be incorporated as well. I have used ICRA and SAFESURF for years now, because they are more effective than posting a 18 Only graphic on every page.

The real issue is that whatever system or rating that anyone comes up with, is only going to be as effective as the parents desire to use it is. I know that the simple parental controls on zonealarm and my cable provided firewall is very effective in preventing access to my site when the filters are on, thanks in large part to ICRA.

I agree more diligence is needed, but there comes a point when we can only do so much. As webmasters placing up a graphic saying 21+ isn't going to end or effect one bit the number of kids who access online porn. The true answers really lies in using the systems in place and in having parents enable those filters, not shut them off.

but just me.
Ian