Working with a rescue the hardest part is determing the dogs age. We generally get an estimate from a vet that checks the teeth to try and determine.
The probelm is that since Yorkies are prone to bad teeth and if rescued from a mill (with absolutely no care) the teeth are much worse which really goofs up trying to determine the age.
We have two that we just have guesstimate ages on...we celebrate their bdays on their adoption day!