The last all ages party I went to, a 17 year old got naked, yes completely naked, and was just chilling. When the promoters tried to talk some sense in him, he ended up freaking out and punching a police officer in the face. Seriously. Now I'm not saying that most 17 years would do that, but the simple fact remains that it would be way less likely to happen with a 25 year old. That promotion company now does 18 and up. Way less liabilities, way more fun.
It's the same reason I would rather go to a bar that was 25 and up. Whether you like it or not, in general - with age comes maturity. I know a ton of 16 year olds would tell me how mature they are, and I get it... I felt the same way, but I guarantee every single one of you, that in ten years you will laugh at that statement. I'm not saying they shouldn't be allowed to go to raves, as far as I'm concerned, that's between you and your parents, but I just don't want to be there. Sorry.
Oh, and when I see 13, 14 year old girls in padded bras they can't fill yet, it makes me want to vomit.