this really seems to be a Youtube issue. I can use Airplay to stream from my Mac (2013, El Capitan latest) or iPhone 6 to my new Airport Express that is connected to my stereo. All works perfect and Youtube even shows the Airplay icon but if I click on it the menu items are grey and can't be clicked. Of course I can still stream Youtube from my Mac by using this method: http://is.gd/8TqA4Y
Anyway, it is not a perfect solution since all audio output from my Mac is sent to my stereo.
If Youtube Airplay would work properly only the Youtube sound would by streamed while other audio signals would still come out of my Mac's speakers.
I will try to report this issue to Google again...