YouTube has a tool - YouTube Insights for Audience - that lets you check out which sorts of people use YouTube, and what sorts of videos particular subsections watch.

So by checking UK viewers only, we can see their age ranges (40% are aged 18-24, for instance) and gender split (72.7% of UK viewers are male).

Age and gender split of UK's YouTube audience

Or you can compare the age distribution of UK male and UK female users.

Age splits of male and female users

You can also see what sorts of videos different users are interested in and what they search for. UK men want Lady Gaga Telephone. The women want the same, just not as much.

The tool doesn't give any clue where the data is from, and some of it conflicts with YouTube's stated demographic data (EG YouTube's factsheet claims its user base is "evenly divided between males and females"). Thanks to Ian Miller for pointing out the tool.

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