I personally will RP in a RP that's not 18+, but if I was ever making one, I think I would enforce an 18+ rule. You can always lie and say that you are 18+ when you're not, and as long as you're acting that way, I'll never really know. However, even though in the world of the internet where a 14 year old can look at all the porn he wants by clicking "yes, I'm over 18" on a screen, I've noticed that in the world of RP it seemingly tends to keep the younger population at bay.
All it really does is give one peace of mind that they can have a RP with mature discussion and need not worry about Pimpleface McYoungbones coming in and ruining things. This isn't to say all people that aren't eighteen are like this, but in my experience, a lot of the "shitters" tend to be 15-21.
Yes, 15 through 21.
Shitters can be of any age, but if you can select a quick option that gives you the belief, however wrong it may be, that you're cutting a big portion of them out, why not? However, when it comes to RPs with violence, I'd never put in a 18+ tag alone for that. Are you really going to sit there and tell me that Little Jimmy over here can watch people getting their heads chopped off on television but he can't write about people getting their heads chopped off on a site dedicated to writing fictional RP? Get real.
*cough*sorryformyrant*cough*
EDIT: But hey, while I'm at it, I personally would seek out RPs with the 18+ tag because those tend to be RPs with themes I enjoy. I do enjoy darker themed RP, and they tend to be the ones with the 18+ tag. While I'm not saying those without them aren't, not by a long shot, I do notice how a lot of the ones without the tag tend to be Dank Weeaboo Fantasy Extreme, LOLXDSORANDOM: The RP. The RPs that I particularly enjoy, just tend to have the 18+ tag on them.
With that being said, I enjoy this one a lot so far. I knew I would, but that's also because I know Squee and he and I go pretty far back.