breeding my shih tzu

Question
what is the best age to start breeding my bitch

Answer
Not until she is at least 2 years old.  She must complete 1 heat cycle and must be over 1 year old before AKC acknowledges the litter.  But for the benefit of the bitch and maturity, you really should give her at least 2 heat cycles and wait until you can accurately calculate her rhythm and get her through the puppy stage to see what type of mother material you have.  It will also help you be fully prepared as well.