All you have to do is put "you must be under 18 to solve this captcha" built into the captcha itself. Anyone on the porn site is over 18 because that's the law, and then they won't be allowed to solve the captcha because they are too old*!

* does not work in areas of the world where you can view porn under 18, or on anyone who lies about their age, or any porn sites where there isn't an age requirement, and it means your site won't allow any users over the age of 18 to contribute
hey, it's not perfect