I would say the situation is mixed here in Paris. I'm 31 and have been going on a regular basis to jazz related concerts for five years, much less regularly before where I used to see more live rock. Some very commited older jazz fans always get the front seats and rarely miss a single concert for big names in free improvisation. But there is also generally a group of 20-30 year old people that come later and stands or sits behind, who might be less knowledgeable about the music's history but who enjoy it just as much. Some of them are musicians, others come also to hang out. The prices are more than reasonable for younger audiences, generally from 5 to 15 euros per concert. At one time, groups like Sonic Youth had an influence in bringing a part of their audience to jazz and there came a lot of dedicated fans out of that stream. I agree though that the conservative aspect of some jazz halls and current traditions can have a repelling effect, it is also the most visible part for people who have only a vague idea of the dynamics of the music. I started going to concerts when I discovered places like the Instants chavirés and stopped thinking that jazz was at the rue des Lombards to give a parisian example.