large breed dogs can come in heat for the first time as early as 8 months old and have puppies. I strongly discourage anyone from allowing this to happen as they are still very much a pup themselves and their body has yet to grow. Unless she is exceptional quality per the breed standard I would have her spayed before she reaches the age to start her heat cycles
I would have a vet give her a check up they can tell you an approximate age on her by looking at her teeth, however it will be an approximation. She is a very pretty dog but I still would not breed her especially without knowing her history. GSD's have a predispositon to many health problems mostly hip dysplasia. No matter how cute they are as babies, they do grow up and if there is a history of dysplasia or any other inherited problem it's passed on to the babies which means a litter of puppies that could have serious health issues as they get older and the owners will be forced to pay out hundreds of dollars in vet expenses, not to mention the suffering the dog will have.
Breeding dogs is not something I take lightly, there are so many things that can go wrong. I have 2 dogs that were born with demodex mange although the breeders insisted their dam does not have it..news flash she does or these two which are a corgi & a chihuahua would not have it period. Also being a large breed dog she will have a lot of puppies she could have as many as 10 that's ten more dogs on this earth that may have health problems and you are responsible for.
Another thing I have to ask is do you keep her chained like that all the time? A chain wrapped around a dogs neck can embed itself in their neck if you have never seen it happen I will tell you it is horrific and very very painful. I have seen this on more than one occasion it is not something you ever want to see believe me. Not only that the chain will eventually rub the hair off on the underside of her neck and it will be very sore. If you must tie her out get her a collar that fits her well asap for her own safety.