That's why all the shows are all ages shows. It depends where you go though. If you go to a show in Jersey, it's all young kids. IF you go up to a show in the city, you see a lot of older guys. It all started in the 80's with bands like Black Flag, Bad Brains, Minor Threat.
Older guys come from that generation.
It's not mainstream, I don't know any mainstream hardcore bands. It's tricky though. Some bands will come form the hardcore scene but not sound hardcore. They may sound mainstream, but they still aren't. Like Lifetime , they were brought up in the hardcore scene with hardcore fans.