I did a bunch of research on this last night. I've wondered about it too. Here's what I came up with:
The U.S. federal statute on the distribution of obscene material over the internet gives 18 as the minimum age, which means that no state or municipality can allow anything less than that. However, they can require more. Do they? No. I've checked every single state and they all list 18 as the minimum age.
However, a couple states do it a little oddly, and I think this is where the "or 21" comes from. The states where this might be an issue are Alabama, Nebraska, and Mississippi. All three of these states define their "age of majority" (aka, the age at which you are no longer considered a minor) as something older than 18. In Alabama and Nebraska, its 19. In Mississippi, its 21.
All three of these states specifically list in their obscenity statutes that you cannot distribute obscene material to anyone under the age of 18. However, if a municipality chose to piggyback the "definition of terms" in their local ordinances off of the state code (which many do) and then chose to write their own local obscenity law (which many do), they might end up writing it as "you cannot distribute obscene material to a minor." In which case they would have unwittingly written a local ordinance that had a higher age than the state statute.
And, of course, there's always the possibility of municipalities who choose to raise the bar higher than the state code anyway. However, I've checked around and haven't been able to find a single one that intentionally does so. This is mainly because most municipalities borrow most of their language from the state code or from other larger municipalities so that they don't end up accidentally writing something that is unconstitutional (and its just easier that way).
If you're concerned about your specific municipality, you can try
MuniCode. They keep a publicly accessible library of the municipal codes for hundreds of cities and suburbs around the US. Just find your city, type in a search for "obscene," and then look for the ordinance pertaining to distribution/dissemination to minors.