Members of the breed are not usually ridden until they are four years old, and structural development is not complete until age seven. Their most productive years are between eight and eighteen, although they retain their strength and stamina into their twenties. An Icelandic mare that lived in Denmark reached a record age of 56, while another horse, living in Great Britain, reached the age of 42. The horses are highly fertile, and both sexes are fit for breeding up to age 25; mares have been recorded giving birth at age 27. The horses tend to not be easily spooked, probably the result of not having any natural predators in their native Iceland. Icelandics tend to be friendly, docile and easy to handle, although also enthusiastic and self-assured. As a result of their isolation from other horses, disease in the breed within Iceland is mostly unknown, except for some kinds of internal parasites. The low prevalence of disease in Iceland is maintained by laws preventing horses exported from the country being returned, and by requiring that all equine equipment taken into the country be either new and unused or fully disinfected. As a result, native horses have no acquired immunity to disease; an outbreak on the island would be likely to be devastating to the breed. This presents problems with showing native Icelandic horses against others of the breed from outside the country, as no livestock of any species can be imported into Iceland, and once horses leave the country they are not allowed to return.