> If it's illegal in the United States to ask someone's age before distributing porn to them online because of the first amendment, why can physical porn stores ask for id? Is that also unconstitutional?

The laws typically don't require them to check ID but instead punish them for selling to minors. You then have several major differences from the online case:

In a physical store where they're neither de facto nor de jure required to check your ID when you're clearly an adult, many of them then don't. There is no feasible way to do the same thing on the internet so instead it effectively becomes requirement to ID everyone, which is different.

In a physical store the clerk can already see your face and hear your voice. It's already hard to be anonymous while interacting in person. A law that compromises anonymity in a context where it was already compromised is different than a law that compromises it in a context where it wasn't.

In a physical store, someone who checks your ID is a human being. They're probably not even going to remember you, are just a store clerk even if they do, and you can see if they try to photocopy your ID or similar and refuse to allow it. Or, you may have a human relationship with that person and trust them not to share your identity with their employer or anyone else. For an online service it's a computer operated by a corporation, and then there is no way for you to tell they're not storing the information, which they have a perverse incentive to do so they can tie all of your future and past interactions with them to your ID. This results in a much stronger chilling effect.

Moreover, a lot of these laws predate the sort of databases that now exist. If someone started making surveillance cameras that could undetectably read the barcode from your government ID if you took it out anywhere in the store and then record it in a database to associate with your activity, your typical defense from that would be to not take it out while you're in the store. At which point a government mandate to show them your ID would have different implications than it did in 1975, which could affect its constitutionality.

It is not illegal to ask a user's age in the US online. Can you let us know what your source is?