truthfully, you'd do better to do demographics for your specific hubs, because each topic you write about attracts a different group of people. Also, you could fore go trying to gauge your demographics and just pick a demographic to write for. Any will do, then you just figure out what that demographic wants to read.
Though, if I had to give an educated guestimate of the demographics for most of hubpages readers on a whole, I would say our readers are between the ages of 25-55, mostly female, mixed races, low to upper income and they like to read. Though that's pretty broad, obviously.