HD, if the dog has it, can be diagnosed at anytime with xrays. A 6 month old puppy can be xrayed and if it has HD,can be diagnosed easily at that time. OFA numbers will only be given at 2 years old, prelims can be given at any age from a puppy on, but will only be posted on the site at 1 year old.
I have had OFA do prelims on a 9 month old Sheltie puppy and was diagnosed with both hips having severe HD. This puppy moved like a dream in the show ring, but I was noticing when he moved, he was lowering his head slightly. I decided to xray him, sent the prelims to OFA and they gave me the diagnosis. He had horrible hips.
I know another dog that won group after group, fabulous movement, could jump, with no issues and moved like a smooth machine. Did her hips when she hit 2 and she had the worst set of hips I have ever seen........
My first GSD at a year old was fine one day, came out of her crate limping slightly one morning and I took her in for xrays. Didn't even send them in, one hip was subluxated out, one had no hip socket to speak of. She never limped until that day. Did FHO on both hips.
What I am trying to say is HD under a year old os not so rare at all. Many many cases of it are dignosed everyday, that is why OFA suggests prelims if you have a concern. A severe case can show up as early as 5 or 6 months.
Don't assume that if your dog doesn't have HD at a year, it will never have it. But, don't assume that a dog under a year old won't have it either.