When I was in high school and college it seemed the people going to the same shows as me were at most slightly older. I remember once going to a show when I was 22 that was all ages but you could get a bracelet to go downstairs to get alcohol if you were over 21...and I'm pretty sure I was the only person present not in a band with a bracelet.

The obvious answer would be that those 40 year olds at shows were 20 year olds going to them 20 years ago....but there was also punk shows in 1984, so why no 20 year olds from those going to punk shows in 2004? It's like people outgrew it then but they don't now.

Also to clarify I am not just talking about tours from bands like Descendants or Bad Religion that have been around that long. I mean like actual underground basement shows or ones in dive bars or at least ones with newer and more well known but still distinctly underground bands (like Mannequin Pussy and Drug Church.)