All dogs mature at fairly similar rate. A dog is considered a "puppy" until it turns 2 that is when they are considered mature dogs. Most dogs hit their "teen years" at about 1.5 years of age. A male can be bread from that point on. For female dogs, it is safer to wait till the 2 yr mark before beginning to breed them, so that their sexual organs are fully developed and ready to carry a litter.
I have a female shih tzu who as come into season for the 1st time, the boy shih tzu I have is 16 months old and they are inseperable. They bonded from day 1 and will not be parted .I'm worried that they will breed when they are alone and don't know how to keep them apart. Would it be dangerous for her ifshe got caught out.
Shih tzus are able to breed once they have their first heat which can be any time from 6/9 months old. It is not recommended that you breed from her till she is at least 2 years old. Males can breed from about the same age but again it is not recommended you do so till he reaches maturity.