It depends on the venue and the style of music as to the age of the audience...
We played a gig at a studios in East London last saturday night as one of three bands all playing original music, one classic style rock, one shoe gazy type stuff and us (folky rocky type stuff) a good 60 odd people watching and I was definitely amongst the oldest there at 32...
The studio puts on these nights (essentially gigs) every month and they are invaribly well attended... however... all those attending, whilst young, are active musicians themselves (who reherse at the studio) and so have an interest in music. Of the gigs we've played at regular music venues the younger folk generally have absolutely zero interest in live music.