when I was a much younger jazz fan, say as an 18 year old in 1999, I was always uncomfortable at jazz concerts with everyone being so much older, even the occasional university student being a decade older and the majority being elderly gentleman with beards... I'd say it was like that at most jazz venues, not just avantgarde (in my case back then usually the Loft in Cologne), the major exception being festivals, especially Moers (which always drew thousands of younger people who were just hanging out, smoking and living in their tents without bothering too much about the music...). Now that I am myself a middle-aged gentleman with a slowly whitening beard, I feel like I fit in much better... but yes, those audiences tend to be old... The only real exceptions I see now is when it's artists the jazz students from the nearby convervatories want to see (like Ambrose Akinmusire or Julian Lage)...