How to validate users age for adult sites

There are some sites that ask for a credit card just for validation of age and identity. Other than that, Just asking for the age and having the person enable the checkbox that there are of age and understand the terms and conditions of the site, is the approach that I have seen most implemented.

Just taking the person's word for it is not good enough IMHO. I don't want to break any laws by allowing under-age children access to adult-content. I've started googleing for this and found one promising site that provides that service.

absolutely...i wasnt suggesting that my first response was advice. I would agree that when dealing with age issues, I would most definately seek out legal advice. Keep in mind that if you do find a managed service, still seek out legal advice as the laws from country to country may vary. I am no legal expert at all, but my thoughts are for example, if you are complying with US laws, but this service is based in another country, that can be an issue for you.

this is an interesting topic. I am going to look into this as well.

found one promising site that provides that service

I wonder how they do this. Based on web presence alone, there is no fail proof way to check it.

I tried to do this a while ago and came to the conclusion that all I could practically do was put up a warning, get the user to agree to them (in addition to a clause about age). Muddying the waters with credit card numbers and such is a bit awkward. What about users without CCs? What do you then do with that info? You are legally responsible for storing that info - if you store it. A kid could get their parent's CC anyway.
If you look at Facebook and BBs, they all have a similar approach. I remember phpBB used to (still do?) have a COPPA thingy.

//EDIT

Don't the 'laws' pertain to the country where the server is based?