I think you have to consider what time you play as well. Between 8 and 4 (depending on the time zone) there probably aren't many people under 18 playing because of school. Then there's probably a huge influx of 10 - 18 just after 4. I would venture go guess that the older people come on around dinner time or a little later. Thoughts?
I recently saw a graph on how many people played WoW over the past 24 hours. It peaked just after school ended (or was it around 7 pm, I can't remember).
Oh, and fun fact I found on that site. There are more Warriors than Hunters at 70.