Steam's side of the problem
From Steam's point of view, every user of Steam has to be at least 13, because they positively affirmed to be so when they applied dor a steam account and confirmed Steam's TOS. It's a sentence that can't be bypassed even with parental consent, as it is a hard requirement for the contract to be valid:
You may not become a subscriber if you are under the age of 13. Steam is not intended for children under 13 and Valve will not knowingly collect personal information from children under the age of 13.
That language is as clear as "No admittance below 18" at a night club door, "No alcohol below 21" in some bars, or "no rentals below 25" at a car rental. They don't want to and will not contract with anyone that is below this age. As a direct result, any and all games on steam can be assumed to be rated the equivalent of PG13 (USK 12 in Germany, A in Japan, etc.) or below without the need for extra marking: people below 13 are not allowed to use steam, not even with parent permission.
Now, games for older people are behind an age verification barrier. To access them you have to positively confirm that you are above 18 by entering your age. By this measure, Steam tries to set up an extra gate. For example, "The Outer Worlds" (the game is rated USK 16) does prompt:
THIS GAME MAY CONTAIN CONTENT NOT APPROPRIATE FOR ALL AGES,
OR MAY NOT BE APPROPRIATE FOR VIEWING AT WORK.
The developers describe the content like this:
“This Game may contain content not appropriate for all ages, or may not be appropriate for viewing at work. This may include: Blood and Gore Intense Violence Strong Language”
By setting your date of birth and deciding to view this page, you both confirm that you are of the required age (for your jurisdiction) and that you are agreeing to view this content. It's like a door in a shop that reads "Beyond this point is the adult stuff. By going up there, you agree that you want to see this, and to get in you need to tell us when you are born."
However, it is more than just a simple age check: Using for example a 2008 date (=12) results in "Sorry, but you're not permitted to view these materials at this time" and trying again either by going back or reloading the page does repeat the error: it does remember you (for some time). So it is a somewhat effective age barrier - as the error page does prompt "retry" and then allows you to take the test again. But the casual kid is deterred. If you used 2003 instead (=17), the game appears from Germany, as you are older than the USK 16 demands.
So, from Steam's PoV they did their diligent duty to prevent people from accessing material that they might not want to be subjected to and which they are not allowed to buy in some jurisdictions. Why?
Why does Steam need Age verification at all?
Well, Steam is multinational, so First Amendment is much less of a concern for them in this regard as they can't use that defense anywhere but in the US. As they are a retailer, they can contract with whoever they want, as long as they follow the law wherever the contract is made - both for selling the game and agreeing to sell it.
As a retailer of games that sells games internationally, they have to comply with the law in all jurisdictions where a game is sold. So while the First Amendment allows the game maker to not mark the game in the US, Steam on the other hand will need to comply with local laws in other countries.
For example, the German Jugendschutzgesez (Law for the protection of the Youth) prohibits making a game accessible to someone that is below the USK-rated age. So a USK 18-rated game accessible can't be legally bought or downloaded by someone below 18. Also, you can't advertise for a game that does not have any USK-rating at all, so it's pretty much impossible to sell and distribute a game that does not have a USK sticker in Germany, making USK-rating factually mandatory. For example, Wolfenstein II and Fallout 3 are USK 18 and the game pages do carry the red USK sticker on Steam - and in the case of Fallout 3, there are multiple versions - all of them rated separately by the USK to need the 18 stickers, though only the very first one has been cut in violent content.
Another example is the Japanese Fallout 3 version which does not contain one quest and its accompanying NPC as well as some animations because the first two would hurt the sensibility of the only country that was ever nuked in an act of war, all might fall under the obscenity laws (Article 175 of the Criminal Code (1907) of Japan) and as a result, CERO could block the release in the country otherwise. CERO still rated the game Z, the equivalent of USK18 in Germany. As a result, it's illegal to sell the game to someone below 18 in Japan - and to buy it if you are under 18.
（わいせつ物頒布等）(Distribution of Obscene Objects)
Article 175 A person who distributes, sells or displays in public an obscene document, drawing or other objects shall be punished by imprisonment with work for not more than 2 years, a fine of not more than 2,500,000 yen or a petty fine. The same shall apply to a person who possesses the same for the purpose of sale.
Note that the Japanese law does not define what obscene is, but it follows the "We know it if we see it" rule.
Publisher side: How can Steam require being informed about content and age ratings?
Well, it's easy: To put a game onto Steam, you sign a contract with Valve. Even the informational page about the contract contains this clause:
What you shouldn’t publish on Steam:
- Adult content that isn’t appropriately labeled and age-gated
After applying, you'll get a contract that contains clauses that include your obligations that fit this, and they include - as far as I was told - that you have to tell them about the ratings you got in various jurisdictions (among them: Germany) and to inform them about any and all mature content in such a way that you need to label your game properly and activate age-gating.
You can't use the First Amendment outside of the United States, as it is not applicable Law outside of the US. You can't use the First Amendment against a contract, because you agreed to do something. And finally, you can't use the First Amendment against a private entity unless it performs a governmental function, and Steam doesn't do that.
If you don't want to do your part under the Steam publishing contract as a game developer, you shall not sign the contract, and as a result, you will not be able to publish your game on Steam.