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Thats been a hot topic on the internet for decades by now. 😉
I honestly dont know about any good ways to check up on American ages.
Over here in Germany we got a service called "Postident". (where "Post" means "Mail")
The customer (Websiteuser) prints out a form with their personal data, which is also send to the post service and takes a print out to their local post office.
The local postman checks up the data with the customers identification card and enters some additional data from it.
Then the local post service omits the validation to the website administrator and bills them.
The fees for this validation are between 3,00€ up to 8,40€ depending on the amount of verifications per month (the more, the cheaper) and if the data gets omitted to the website owner via internet or mail service.
In the end its complicated for the user (as they have to wait for the verification to arrive and be processed by the site owner, additionally to the limited service times of the local postal offices) and way too expensive for small website owners.
If you ask me, a simple date-verification is enough.
In germany one will have to do some additional checkboxes (kinda like "I have read the terms and hereby affidavit that the data I enteres is correct and I will make sure that no minor will gain acces this way") to be on the law-safe-side.
Generally people does not declare fake data in famous social networks like Facebook, Google+ or Twitter, because they use them as a social tool.
You can integrate your registration with Facebook, Google or Twitter APIs and require a valid account where the date of birth is older than 17/18/21 years, depending in which country you are.
Well, taking me as an example, I don't use Facebook or Twitter and have not entered more than absolutely necessary on Google+.
I see a point here when content is "explicit", but then again what works in one country will not work in another one, some countries use credit card numbers or hash ID card numbers for the encoded age, but not everyone is willing to hand out these information, and it's neither any good nor working everywhere.
So, as long as written content - providing it is not illegal to start with - does not have an age restriction in the local bookstore, I see little point in harassing users. A one-time popup explaining the explicit nature of the content and the confirmation of "yes, I know what's coming" ought to do.
