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Pornhub goes dark in Arkansas after age verification law kicks in
(www.theverge.com)
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The government doesn't go out of it's way to give you one, but they usually aren't difficult to get. Driver's licenses and passports are commonly used as ID. Many states will also issue a state ID card, though the process for getting one varries by state.
Driving, or at least being able to drive, is so ubiquitous that nearly everyone over 16 has some kind of driver's license. That's especially true of rural areas like Arkansas.
For these kinds of things "official document" typically means a driver's license, passport, state ID, military ID, etc. Anything issued by a state or federal government that has your name, date of birth, and photo.