There is a reason why web sites ask you for your age, gender, and zip code – the ability to target advertising based on demographic data. Now, YouTube is putting that data in the hands of video producers, allowing you to see on a video-by-video basis the demographic breakdown of your viewers.
For now, you get a bar chart representing the percentage of your audience in different age groups, and a pie chart showing the breakdown of gender. You can then filter that data by country or for a specific time frame. Additionally, YouTube has rolled out aggregate data, so you can see the combined demographics for all of your videos. The data is all anonymous.
Given YouTube’s dominance in online video, the stats they provide are likely to be seen as fairly authoritative, hence giving video producers a better idea of what type of content they should be producing and advertisers a better idea of what they’re buying.