What age is best for putting a dog up for stud surely not before health tests are done

Agree, certainly not until those are done and the dog is mature enough to assess his qualities as an adult.

Yes, agree with these and the other thing that might be considered is his age in terms of his achievements.  That's probably not phrased correctly, sorry.  I don't have show dogs, mine are all workers, but I do know that once a dog has been used at stud he becomes more 'bitchy' and by that I mean he sniffs a lot more, tons more interest in other dogs and does more scent marking - this can be a problem if competing (any form of competition really where you don't want the dog to be doing the above) with him.  I know from what other people tell me, and my own experience, that after a mating it can take sometimes just days but more often weeks and months for the lad to get his head back into gear! Anyway, just another angle, if you can work out what I mean :-)