13. For the purposes of paragraph 4 of section 12, a disability is severe only if by reason thereof the person is regularly incapable of holding any substantially gainful occupation.
However, in the case of a person 60 years of age or over, a disability is severe if by reason thereof the person is regularly incapable of carrying on the substantially gainful occupation the person held at the time he or she ceased to work owing to the disability.
A disability is permanent only if it is likely to result in death or to be of indefinite duration.