18+ concerts? What's that?
Shows you can only attend if you're 18 or older. I know they exist in the UK too, and the US has 21+ shows. It's due to liquor licensing laws. At venues that are licensed liquor establishments, e.g. clubs, they are not allowed to admit anybody under the legal drinking age to any concerts held there. The only time they hold under 18s shows are when absolutely no alcohol is being sold that night inside the venue.
Obviously, this doesn't apply to larger venues, like sporting arenas or stadiums, where people are carded before buying alcohol. It's only for establishments where the primary business function is selling alcohol, which tend to be the clubs that have a band room and a bar, where smaller acts perform.