> How old should a dog be?
Do you mean how old before using at stud or how old for hip scoring? Hips can be done after 12 months
I have no direct breeding knowledge other than to say that the majority of dogs used at stud in my breed are 3 years plus. Although an experienced breeder may be able to tell how a dog is progressing, particularly if they own and breed family lines, it just isn't always easy to see how an immature dog will develop. I think quite a lot of people might report that promising youngsters don't always fulfill their early potential either in look or temperament.
I don't own bitches but if it were me I want an experienced dog with an even more experienced handler. I'd want a dog that I watched from a puppy in the ring and one from whom I've seen other progeny. I wouldn't risk the health of my bitch to anything less than a tried and tested stud.
If you are a novice I'm sure it pays to have as much experience as possible in the mix so it's well worth finding a mentor to advise you. People on here will recommend some books too but nothing can replace someone with good breed knowledge.