In my experience I would say that the majority of people are from 55 to 75, with around 60/70 % being women.
Having said that, this is changing (quite fast I think), with more young people and a much more even male/female ratio. Ben is often asked the age range on particular holidays, and if you look at his replies, both of these trends are clear. It will be interesting to see how this affects the type of holidays offered by JY.
At 64 you would be bang in the middle, but age is only a number anyway, and it's more about your approach to life/attitutes than physical age. Thus I've met "very young" people in their late 70's and "very old" people in their 30's on JY holidays !
Stuart