Ok, so the plugin you're using (this one? https://github.com/danwilson/google-analytics-plugin) doesn't support this out of the box.
You could just fork the repo on Github and add the last line of the code example from that Google support page you posted manually.
For Android you could probably put it here after this line https://github.com/danwilson/google-analytics-plugin/blob/master/android/UniversalAnalyticsPlugin.java#L119 and for iOS ... I have no idea. Sorry, I hate Obj-C.
Better would be to write a function to enable the demographics stuff, not just force it in the existing code, and then contribute that changes back to the original project via a pull request.
If you can't figure it out yourself, write an issue on the plugin github. Maybe someone else has the same problem and will get around to implementing it.