My QH started having hock issues about 4 or 5 years ago. It started on the left side and x-rays showed at that time it was about 50% fused. It has since become a problem on the right. Last summer I had him looked at again and the left is now about 90% and the right is at about 50-60%.
Some horses never fuse completely
and yes excersise will help with the fusing, but only up to a point. There are lots of articles written by lameness vets on this subject, as it's become so common. Even at 90% the horse can still be productive but he might need to have his "job description" changed. A HJ may have to be down graded to something else.
I used to jump with my guy and now we work on more dressage. He's still my trail horse and has no problems with going on a 10-14K ride with a little trot and canter thrown in. As he gets older the winters are a little harder on him, especially as we live in a cold damp climate. He gets lots of turn out, I try to keep his weight down, have a chiro look at him 2 or 3 times a year to make sure he isn't compensating anywhere else in his body from any discomfort in his hocks and if required he is put on a low dose of bute. (1 gram every other day). Hock injections are also an option.
I vetted my guy before I bought him and he failed the flexion test on his left hind. At the time he had never taken a lame step. X-ray's at the time (10 Years ago) never showed anything. Knowing what I know now would I still buy him? Yes, I have no problem changing my priorities to suit how my horse feels.
I suppose if competing was the bee all a person may feel differently.