Why would a 8 to 9 month old Yorkie have brown plaqe on her teeth ? I'm asking for a friend. She eats hard food, but I don't think she gives her chew toys (raw hide, bones, etc....)
Maybe the quality of food isn't that great, or she needs something hard to chew on. Regardless, your friend should still be brushing her teeth once a week or more.
When I seen her she gave me kisses and of course a got a wiff of the breath which wasn't so fresh so I peeked at her teeth and her (sorry I don't know the tech term for them) pointy long teeth on the side had the brown build up.