Apart from those number not making much sense and being made up, there obviously has to be some limit if the plan is to go drinking. TomB's idea seems fairest, because it lets under 18s attend and meet everyone, then the people who are 18 or over who want to go drinking can do so later. And to be honest i'm not sure how many under 18s will be able to travel to it if it isn't near them (it probably makes sense to have it in a capital or major city which it's cheap to get to and stay in, and has good transport and which people would actually like to visit).