Steam – Why does steam keep asking if I’m over 18

Of course you could run Steam behind a proxy server, provided that the administrator opens the necessary ports.

You can find a good guide directly on Steam Support: Required Ports for Steam.

Basically in order to just run steam you need:

Steam Client

* TCP 27014 to 27050 inclusive (Steam downloads)
* UDP 4380

Steamworks P2P Networking and Steam Voice Chat

* UDP 3478 (Outbound)
* UDP 4379 (Outbound)
* UDP 4380 (Outbound)

Than it really depends on which multiplayer game you want to play. In this case additional ports opening is required.

Instead, if you want to play only single player games, the steps before should work in 95% of games except those that requires additional connection requirements, like, for example, some Ubisoft games, some Game For Windows Live (GTA IV), etc.

Steam shows the ESRB rating in the Game Details section on the left sidebar on the games page.

Here's Bejeweled 3's rating:

Duke Nukem Forever:

At first, I suspected that you don't see it when you go to view Portal 2 because you already own the game. Indeed, I own it too and do not see it. Since the ESRB only requires that you see the rating during the purchase process, not showing it once you own the game is fine.

From the ESRB guidelines:

The assigned rating icon, content descriptor(s) and if required, the online rating notice must be displayed on a webpage or digital delivery service screen prior to consumers' purchase/access/download in a location where they will be accurately associated with the game and cannot be bypassed.

Since it's not required after purchase, I figured Steam was removing this extra information since it wasn't needed. On a hunch, however, I went to view a Valve game I did not own to confirm (Left 4 Dead). The rating is not there.

Perplexed, I went to view a non-Valve game I do own (Defense Grid: Awakening). The ESRB logo is there:

Further searching of my games list confirms that, for some reason I cannot explain at this time, Steam does not list the ESRB rating for Valve games.

While the ESRB is self-regulatory and technically voluntary, the fact that Valve's games are rated by the ESRB means they were submitted for rating. This means that Valve is opting in to ESRB participation. It's possible there's some rule that allows them not to post the rating before purchase, but it's not one of which I am aware. I went to "buy" Left 4 Dead (stopping before the final purchase) and I didn't come across a rating at any point in the process.