What's up with age gates on websites? Are they a legal requirement? Why do some just have 'click here if you're over eighteen', but some make you enter a full date of birth? There's really strange things like Steam, where you have to enter despite being logged in, you don't even need to have the date there match the date on your account.
Ahhh age gates. It's like this:
First of all, the law of the land the server is in is what dictates the applied law. Sounds trivial, but we're in internet land, so it isn't.
Age gates are not a legal requirement per se in the US, which is probably what you're referring to (although there's a powerful politic entity that can de-facto force you to implement age gates if they've got you in their sight: The ESRB, which has the power to fine you if your age gate does not meet their rating; basically the same situation as with swearing in broadcasts and the FCC in the US). Anyway, publishers demand age gates by default because they abide to ESRB ratings. They have an interest in advertising to appropriate ages, or at least seeming to have an interest (they have an interest in seeming like they have an interest in advertising to appropriate ages, intense!). Reasons for this interest are numerous. There are the ESRB fines, potential advertising disruption, political considerations, different countries to consider, a standard to live by, all that.
Now, the sites that actually show the trailers have to be on good grounds with publishers when it comes to advertising material. Otherwise, they won't get press copies and shit. And they have to comply with the ESRB ratings because the repercussions could be substantial. So they implement a trivial age gate.
Steam probably did exactly that: They implemented a trivial age gate. They didn't bother with implementing the special case that goes like "Is user logged in? Yes: Is user over age? Yes: Skip age gate" because it's not required by anyone who matters, and users themselves obviously don't mind (after all, the age gates are trivial and just a pro forma, so fuck spending more time than necessary on that shit).
HOWEVER. There are scenarios where the law actually REQUIRES _functioning_ age gates that require registration with some sort of ID. One of these is porn on sites that are served from Germany. Interestingly, German enthusiast game sites were straight-out shut down by the government for a while (and maybe still are on occasion) when they just embedded Gametrailers clips, with age gate and all, because the age gate does not meet legal requirements.
Honestly, speaking as an adult, I _want_ proper age gates everywhere, with registration and all, to get over this bullshit that 18+ games can't be advertised over here because of whateverthefuck with the kids. I'm fed up with this idiotic situation that you still can't purchase Doom on XBOX Live over here although you're legally entitled to obtain a copy.
But legit age gates are not trivial in the least, and most importantly, registering means opening your personal data up for profiling by anyone involved, which makes it almost impossible to figure that out reliably while following privacy laws (and common sense).
So if you read all that, you might've concluded that Steam trailers are either served from the US in Germany if they're age-gated, or they're not following the letter of the law of the land. At some point, the absurdity of the whole thing will lead to a proper framework for this stuff, but we're not there yet. Give it another 20 years.