is not a bad thing.
My yorkie is also 7 months old as of the 3rd of this month. He only barks when one of my other three dogs(poodles) have a toy he wants, at which point he will bark continuously until they walk away from it and leave it for him.
He doesn't bark when the other 3 are going crazy when someone comes to the door or at the fire siren at which the other 3 howl. Their barking has no influence on him at all. He is a nice quiet little fellow and I gotta tell ya I love it that way.
I too have wondered if this is normal as I have heard others complain about their yorkies incessant barking, I have one poodle that barks continuously and I have to tell you it is very annoying, I believe he was possible inbred as he is very high strung, it isnt' just a poodle thing because I have had other poodles including the other two I have now and this is not their normal demeanor.
I say be happy with a quiet companion!