@hentaispyder  
I see your point, but glass isn’t the best comparison. Glass is the exact opposite of pliable. It doesn’t bend and just breaks if you apply enough pressure. You can’t stick a needle in it.  
I think a better comparison would be a crocodile’s hide. It’s thick, it’s tough, but it’s not invulnerable. A crocodile won’t be bothered if you stick something a few millimeters into its hide, but will feel pain if you go too deep. Though crocodiles have tougher skin than humans, with an object sharp enough you can still pierce it with relative ease.  
It hasn’t been explicitly stated in the show, but I think it is safe to assume that the scales of an adult dragon are much tougher than a baby’s, so you’d probably be right about them.

@hentaispyder  
Yes, you can see the green blood on his scales in Owl’s Well.

@QueenCold
 
@Background Pony #51B1  
Did either of those draw blood? You can stick a needle in something but that doesn’t mean it will pierce it. You can also scratch and scuff glass.
 
But anyway, I saw this and thought someone made a “Twilight plays Surgeon Simulator” video.

@Background Pony #51B1
 
Yes, people seem to be confusing thick with impenetrable, which they are clearly not.

@QueenCold  
He got scratched up by chicken talons, too.

@Greylander  
You can stick sewing pins in his scales.

I think you’ll need more than a scalpel and a syringe to pierce those thick scales!
 
A crowbar, perhaps.