IPhone 5 won't play youtube when phone is locked?

I listen to music via youtube but my iPhone 5 does not let me play it when my phone is locked. I'm aware of how you are supposed to do it (double click home button then click play) but it just plays songs from my music library? My cousins phone does it so why doesn't mine we have the same phone aswell.

Try plugging your iphone into the computer making sure your software is up to date!

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