Re: how to require at least 18 years old for signup?
charge membership with a credit card.
Require members to enter birthday, if they have real friends, let another member “validate” their birthday.
have people uploading avatars of themselves holding up signs for your site. This is pretty low tech, but most people can handle it.
There is a level of animosity which the internet provides, and is often hard to penetrate.
You can also protect yourself, by registering your site with known parental control software packages, such as mcafee. This would probably get your domain, and possibly your server marked as adult so probably not the advised solution.