That is another good way to weed out those who are health testing from those who just say they are. If one is advertizing an 18-month-old stud and saying he's health-tested, they are flat-out lying. Some big-name breeders are hooking up their females at that age, too, because they know they can get away without health-testing due to their reputation. You can prelim a puppy, but you're not going to get true results until that dog is 2 years old. If they're healthy, they've got a lot of good breeding years in them, so its stupid and greedy to go breeding them so young.
I recommend digging. Passing health test results go into the public OFA database, and you can look them up. Sometimes you have to sift a little bit because of their search engine. If the breeder is sensible, they will provide you with a direct link where you can see it. Beware the breeder who just does one or two of the more obscure tests (cardiac, patella), because most likely they're just looking for the stuff their dog has the highest chance of passing.
Pony up those results, breeders. Gravity APBTs - Loki
/// OFA: Display OFA Records