Hi Eddy;
First I would like to say, "Welcome to YT".
I want you to know, I am responding to your questions and request for advice.
I believe if one wants to breed, they have done their research. They accent the breed to breed good looking, close to standard as possilbe. It is no ones business but their own. Everyone has that right.
I as a small Breeder I start my Dam's at about 20 - 24 months. Yorkie's are Toy Dogs, it takes them longer to mature. 20 months is a good age to start breeding. The Dam's are completely grown and at that age you will know if they have any major illness or hereditary problems. I am sure there are some that are not detectable until alot older.
I just recently retired my oldest at the age of 6.
This decision was helped with advice from YT members here and my Vet.
He made sense.
After the age of 6 a pregnancy will take alot of Calcium from the mothers Bones and will bring out minor hidden problems.
He also stated, "If a Dam can produce 3 - 4 Healthy Litters, don't you think she did her job for you?"
Yes there are puppy mills that produce lots more and breed them in their latter years. But look at the poor Dam's health, her bones and teeth especially.
Good luck on all your decisions. I am sure you will make the right ones.
Your here asking, and that there shows alot of responsibility.