The first proper gigging band I was in whilst at school (16-18) usually only made money on the door take. We found our most profitable gigs were playing Saturday afternoons, in one particular bar that wasn't too strict about age.
We could usually muster a crowd of over 50 people, by getting friends and so forth along. Sometimes the bar staff would chuck out a load of obvious under-18s, most of whom would come back round the front and pay us on the door to get in again
If it was raining we'd get up to 150 kids in, and would jack up the price in anticipation...those were the most profitable gigs we ever played!
I very much doubt that'd happen today - all the decent independent bars and pubs seem to have disappeared or become part of corporate chains, and so many more places in town seem to have bouncers these days. Plus I notice bar staff asking for ID from folks I'd take to be early 20s...though I guess that's partly a sign of getting old.
It's no wonder there's so much complaining about the 'binge drinking' culture - seems like by denying younger kids the chance to have a few drinks on a weekend, they save all there pent-up desire for when they hit 18, then just go off on one for a few years.
I'm sure this must negatively affect the ability of young bands to just go out and play new music diy-style; doing your own promotion, building a PA from nothing and learning how to do sound etc.
It amazes me in the more corporate music venues to see dreadful bands playing in room under 100 capacity, yet with full house PA, drums miked up, blasting an awful sound so loud that if there were any dynamics in the music, you'd never hear them.