
If appropriate for the breed, certainly not until all the things the breed suffers from, where applicable, have been tested and the dog found to be 'unaffected' - good hip and elbow scores etc.
Many experienced breeders in my main breed actually use an intended stud dog for the first time at around a year (testing apart), on one of their own suitable bitches, not a maiden one either to see what he might throw. And then he'd be put away (concentrate on a show career) until he's more mature. Being allowed to mate younger rather than older should mean he's fit enough to be able to do the job! It's something else that many males need to be trained for. Which is why the first experience should be with a kind and accommodating bitch.

Many breeders don't let their stud dogs be used at public stud however and if they do, it's usually 'to approved bitches only' which means the owner knows that the mating should produce worth-while offspring. If things go wrong, Sods Law it's always 'the stud dog's fault'

Remembering that the owner of the stud dog is 50% as responsible for any offspring he sires, as the owner of the bitch is.
When I had stud dogs, I turned away more enquiries, than I agreed to. I certanly didn't want my hard work building up a recognisably good line, wrecked by a mating to an unsuitable bitch or my stud dogs associated with poor offspring!