The reason for the differing information is because of the differing organizations that do testing. I am not sure about testing in Australia but in the US there are two different organizations that do hip testing, as far as I know only one that does elbows. OFA the organization that does both hips and elbows in the US requires that the dog be at least 24 months old before they will issue a registration number but you can get a preliminary evaluation after 12 months of age. PennHip which I believe is more like the system in Australia, does their hip evaluations as early as, I believe, 4 months old.
Actually, OFA will do a preliminary eval as early as four months of age…but you can't get a published preliminary rating until 12 mos; and as you stated no OFA number until 24 mos.