Well, yes, it sucks if gigs are in such venues. But I like to avoid the bars that are crowded with 18-year-olds on weekends and try and find something better. Ha, usually that means just going and passing out at Club PRKL.
In countries where the age limit is lower you'll find that each bar has its own crowd anyway, regardless of the age limit. I often go out in a town where the age limits are the same in every bar and you'll find the 15/16 year-olds in one bar and the 30+ crowd in the other.
By the way, my (then) girlfriend visited me with her dad and 15 year-old brother when I was living in Helsinki. It was nearly impossible to find a bar to have a drink together at 8 PM already, which I found quite ridiculous considering the fact that his dad was accompanying him. Fortunately I knew the barman of The Base and he let him stay until 10, otherwise I would have had to fit them all into my small student apartment every evening.
I'm sticking to my opinion: ridiculous age limits are ridiculous