I was going to make a video on this very topic. IMO look @ your analytics for the answer. A good percentage of my audience is between the ages of 13-17 followed closely by 18-34. What i've been trying to do is upload when I think that the largest part of my audience will be online. Again focusing on the 13-17 ages. Going a step farther I can see that the majority of my views of that age group reside in the united states and further still I can break it down and see what states the majority of my views are coming from (i.e timezones) For me, it's pretty even for EST CST and PST time zones so I try to upload my main video for my channel every day around 4pm EST because that's when I feel like most kids will be getting home from school. I don't know how much of an effect this actually has, but it sounds good on paper.