Really it depends on your host and where your host is. Every host, country, and state has it's own set of rules. As already mentioned, you really should restrict access to registered members only, and set the age limit to 18. Just to be safe as some of the penalties are harsh. Remember, it doesn't matter what country/state you live in, but what country/state your site is being hosted in.
In the United States, your members are responsible for what they say & publish on a site to an extent. This includes registering at your site and proper stating his/her age. So if a member registers and says his/she is 18, and is not, it's on him/her. However in some countries user's that register and lie about his/her age is the site owners fault(go figure), but that's just the way it is. That's why some sites require a credit card or age id, even if it's free.
You also have to consider 3rd party applications you may use or end up using in the future. Some 3rd party apps terms & conditions say that you cannot use the app in affiliation with pornography, including all of google's apps.
Just food for thought.
