usually the cut off for older horse is 15 and sadly the answer is yes after it passed the "older" horse catagory most insurers limit what they cover said "older" horse for. couldnt tell you what as each insurance company is different
Go for petplan, if you insure your horse with them on thier normal policy before he is 20 then they will cover for illness and accident up to 25 years old!
other insurers seem to stop covering for illness (like colic) at 15 years old.