Audience demographics

You find it in the Audience tab next to geography, device type and subscriber status. The bars show which age ranges and which gender contribute most of your watch time, so a small group of heavy watchers can outweigh a larger group who watch one video and leave.

It only covers viewers YouTube can attribute, which makes it a sample rather than a headcount. Read it as a rough shape. It earns its keep when it contradicts you: if you write for people your own age and the chart says 45 to 54, believe the chart.