I think there should be a sticky post, or something prominent in the info page, or something, explaining that the age filter is for legal reasons, and preferably what those legal reasons are. Because I just tried it, and it's not just annoying, it's *insanely annoying*.
It's not one notice. It's endless notices that need to be clicked repeatedly.
See for yourself: log out of dreamwidth and go here: http://fanlore.dreamwidth.org/ and click the Age Filter agreement saying that you want to see the content. My expectation after clicking a link like that is that I'll see the content.
Instead, once you do that, you get taken to the front page, where you see a list of identical posts, each one saying
( You are about to view content that the journal owner has advised should be viewed with discretion. )
-- content you've already agreed that you want to see, so why is it asking you again? (Rhetorical question *g*) So you click one. And you get *another freaking Age Filter* notice, and you have to agree *again* that you want to see the bloody content.
Four clicks total, including three levels of "are you sure you're adult enough to see this content? Really sure? REALLY REALLY SURE?", just to see one post relating to a non-age-restricted wiki; in my case, my own post cheerfully offering invites to this now-intensely-frustrating site.
Clicking away back out to the comm brings up all the (You are about to view content...) posts -- once again, you need to click basically blindly to open one. And then? You get *another* Age Filter notice. AGAIN. Every. Single. Time. you click a link. So add two more layers of content filtering for each post you want to try.
Seriously, describing it doesn't do it justice. Log out and try it for yourself.
This is going to keep non-journal-based people away -- and despite what a lot of LJ-based fandom thinks, fandom isn't remotely restricted to journaling sites. The only way to make this inviting for people who aren't comfortable on journals is to make it as open and accessible as possible; they need to at least be able to read casually without being forced to join -- and having to log in to the site, even under Open ID, basically counts as having to join it -- particularly when the thing they're currently being forced to join is a non-related third-party site. (e.g., imagine if Fanlore had picked Yahoo groups as its platform, and while technically the posts were public, anyone who wasn't specifically logged in to Yahoo itself with a Yahoo or Open ID had to confirm their desire to read each post at least twice. Same thing.)
(Beyond which, there is nothing on any of those notices saying "hey, if you don't want to see these warnings, get an account!" so there's no reason for anyone coming in from outside to think "Oh, if I sign up for this service, I bet I won't see this." It simply looks like the most annoying service on the *planet*.)