Hmmm! I'm guess I'm classed as an "oldie" then!
I think it might depend on what type of jazz you're talking about. Contemporary jazz maybe appeals more to younger people, but the type of jazz I like, ie New Orleans jazz, I guess is followed more by "older" people, some even older than me
Having said that, I also have to say that I have never known any group of individuals enjoy themselves more, and get more out of life, than "older" jazz enthusiasts. At one of the jazz clubs I go to, there is a lady who is 80 or more who is a superb dancer - she is immaculately dressed, she has a figure that a 20 year old would die for, she has more energy than I've seen in many 20 year olds, and she is out to have a good time. Many of the others at that club are the same - I have often said that they certainly know how to enjoy themselves there and it's great fun watching them.
My theory is that New Orleans jazz encourages "dis-inhibition" (is that a word?

) and older people have fewer qualms about being seen to enjoy themselves. It wouldn't be "cool" to the younger generation, but older (jazz) people don't care what other people think - they just want to get the most out of life. Certainly at the New Orleans jazz festivals I go to, people are really friendly, very outgoing, and totally without the inhibition of wondering what other people might think. Younger people are generally much more conscious of the impression they are making, and regard displays of dis-inhibition as embarrassing. So a New Orleans jazz festival would generally not be their scene.
Thankfully there are some young people on the New Orleans jazz scene who do not conform to stereotypes and get as much enjoyment out of the festivals as the "older" generation. And funnily enough, although the performers in the UK are generally older, most of the overseas performers at the jazz festivals I go to are quite young.
By the way, pianoboe, do you go to jazz concerts locally - did you go to see Enrico Tomasso a couple of weeks ago? PM me if you and your parents go to jazz concerts locally - I've probably bumped into you without knowing!