can anyone tell me how old a dog should be before he is used at stud.

Well, he cant have all his health checks until he's 12 months, then it depends on breed

I would say somewhere between 14 or 18 months at least.  A dog cannot be hipscored until it is 12 months old, adn then you have several weeks before the results are back.  A lot of eye conditions that are not present at birth are not screened for in puppies.

Generally I would want to wait a lot longer than that to use a male as I want to se what he matures into.  Many a promising or outstanding puppy do not mature into the type of adult you would want.  In slow maturing breeds a amture looking puppy may well become to heavy and overdone.

If you are selecting for working ability you need to see the dog prove itself, and that will take time.

Of course just like human beings a pupy is capable of siring a litter, but just as in humans you woudl not encourage a 14 year old boy into fatherhood, but woudl expect them to wait until they are an established adult with a home and career.

Both dogs should be DNA tested for PRA & at least one should be genetically clear.http://www.optigen.com/opt9_test_prcd_pra.html

Other health issues are decribed here http://www.thecockerspanielclub.co.uk/health.htm