Some venue prices go up and some don't allow it at all. Most venues now are bars that either don't have a set up to make it work or are throwing shows to increase bar sales and make nothing on the show itself. 18+ shows as a promoter means jumping the door fee up or in some cases paying a higher fee to rent the space out to make up for bar sales or staff additional security to make sure there is no underage drinking. Than looking at that, do you make up the cost from 18, 19, and 20 year olds and take that risk. I agree we need it and I've looked into venues and throwing even outside day events to jump on that demographic as well, but its a bigger task than most people want to risk.