I would say this could also be characteristic of teenagers along with the non-verbal eye roll. 

2 hours ago, chris191070 said:

Oh I don't know our resident penguin can mutter and leave nasty pecks at the same time

I'm multitalented.    

What are you all doing in this room, muttering about?? If you can't speak clearly get the hell out!!!

**Says grumpy old Steve from his rocking chair waving a cane.**

I've always thought of it more as an old person word when they no longer can hear  

But this guy looks too happy to be me