My experience has been similar to Charlotte's, and the few we've had all recovered and went on to be sound healthy adults. For whatever reasons, we had joint ill for a couple of years in a row - many years back - in 3 or 4 babies. I believe we started with penicillin and gentocin, and quickly progressed to Naxcel which worked quickly. At least one of ours was a couple of months old. Vet did have to flush a couple of joints on one of them, not as big a deal as I supposed it would be.
My vet at the time was also treating a big horse foal for joint ill - he'd been at the clinic for a couple of months and the outlook was not good, so I guess there are lots of variables.
Good luck.
Jan