It depends. Kissing spines varies massively, at 21 it's likely been triggered by something, a loss of posture, possibly related to ageing but many horses have poor posture for all sorts of reasons sadly. Might be hoof balance (see The Equine Documentalist FB/website). So depends on severity, cause, and how well the horse is treated/rehabbed. I think asking it to remain as a RS pony may be pushing it too far, but perhaps with lighter riders...maybe. I would want a good holistic person that could provide a proper postural rehab programme to see the xrays and be involved in decisions involving treatment and prognosis.
I don't know but I'm not sure that a horse which has rehabbed from kissing spines should ever work in a riding school. Are you hoping that a favourite pony will come back to a riding school you go to?
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