Dall sheep with their majestic horns. Horns grow steadily during spring, summer, and early fall. In late fall or winter, horn growth slows and eventually ceases. This start-and-stop growth results in a pattern of rings called annuli which are spaced along the length of the horn, and can help determine age. Dall rams as old as 16 years have been seen, and ewes have been known to reach 19 years of age. Generally, however, a 12 year old sheep is considered quite old.