We have our pugs x rayed once they are over 12 months of age. Obviously, thats going by KC regulations for hips anyway, but for the HV, the extent of the problem cannot be seen until the skeleton is fully grown and that isn't until about 11 - 12 months. You could have one done younger and it be diagnosed with very severe HV, but once it reaches 12 months it may have "grown out of it" (for want of a better phrase!!) to a certain degree so the severity is less so. That is what happened with my own pug who has HV. Or even worse, the problem may not be spotted in a 4 month old pup when infact if you had waited til it had finished growing the HV would be evident...