Luxating patellas can be genetic or from traumatic injury from jumping on and off furniture, etc. The general cut off is one year. Since your Yorkie was diagnosed at seven months, most likely it is genetic.
Listen the the orthopedic vets and get the surgery done sooner rather than later. Dogs with LP's have to compensate for them in their gait which put stress on other joints and leads to debilitating arthritic changes.
LP's usually aren't painful unless they are out of place. It's the arthritis that causes the pain and if you wait for that, surgery may not be as successful or even possible at that point. We have a member on another forum with a five year old Maltese who has extremely bowed front legs from undiagnosed LP's. She has been told it is too late for surgery and this young dog must live with crippling arthritic pain for the rest of his life.
