Their most productive years are between eight and eighteen , although they retain their strength and stamina into their twenties .
Members of the breed are not usually ridden until they are four years old , and structural development is not complete until age seven .
Members ' most productive years are between eight and eighteen , although they retain their strength and stamina into their twenties .
Members of the breed are not usually ridden until they are four years old , and structural development is not complete until age seven .