It is not advised to breed one for the first after the age of 3 years old. It would be a risk now to her at her age. You might want to check out the Central Florida Yorkshire Terrier Club for more information about it all (whelping, age, studding, standard, quality, health issues, testing, etc..) :
Yorkshire Terrier Club of America Affiliate Clubs
I know some members there and they are very nice.
P.S. It is actually healthier for to have a girl spayed before she comes into heat. Reduces certain cancers, etc... So many more benefits.