I would have to go glance at the rulebook but i believe at 3 years of age an ASPC horse can be registered with AMHR if they make the height limit, but remember shetlands are measured at the withers so the horse would have two measurements ..one as a shetland and one as a miniature. For instance, Royal is 39.5'' as a shetland and 37'' as a miniature.
I do believe the age is 3, im sure someone will correct me if i am wrong though
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Is Devon thinking about getting into the shetlands