You can have xrays done around 6 months of age. But the vet doing the positioning and diagnosis should be experienced. My female went to be spayed at 6 months and started limping a day before she went in. The vet took it upon himself to xray her and then told me she had 'severe' HD. I panicked and went for a consult where I took photo's of his xray. The positioning of my pup(under sedation) was horrid, she didn't have HD but Panosteitis. My vet should have had his license pulled for his incompetence.
Don't start worrying or panicking until you actually have a qualifed vet assess your pup.