Ok, now most adult based sites have a global audience. Mine has a mass load from UK, handful from USA, a lot from Europe and some from other countries like South Africa, Australia, etc.. etc.. Now I say on the entrance page that you must be over 18. Now when they sign up I remove any member that has there age as less than that, but what happens if, lets say "Country A" has a pornographic viewing age limit of 21, then do I have to also ban any user that has their country specified as "Country A" that is under 21? Or do / can I just stick to MY countries age law? (Which is obviously 18)
Normally you would ensure that the site complies with the laws of 1) where you/ the company is legally based and 2) where the server is based. So if with your example you are in B with age limit 18 but your server is in A with its 21 min age then the site would have to have a 21 age limit. What you need to do to cover your arse is put the onous of complying with their local laws onto the user and likewise state that your website is governed by the legal juristiction of B. These should feature in the terms and conditions with the entrance page stating that by following the enter link they are agreeing to the t&cs (with a link to the t&cs of cause)
Stick to the law in your locality - a foreign country is unlikely to begin extradition proceedings against you if one of their 20 year old citizens looks at porn a year early.
Make sure that your terms of use reflect that proceedings are under your local law and jurisdiction. Little things sometimes help and the above is quite true. The authorities have a lot bigger fish and problems to deal with than your site - unless it becomes a juggernaut.