In need of an Age gate

Ok, I’m looking at changing the way my site posts news and videos. It’s a video game site and the ESRB wants an age gate before M-rated games are shown. Is there anyway I can add a tag or a category that when checked puts the article behind an age gate (naturally I’d like to have it so that they only have to enter their age once per session or just once and have it remember the IP or user name).