The person adopting a child must be at least fifteen (15) years older than the person adopted, or twenty-five (25) years of age or older, except such age restrictions or requirements shall not apply in cases where the adopting parent is a spouse of a natural parent, and except that such age restrictions or requirements shall not apply when the person adopting an adult shows to the satisfaction of the court that a substantial relationship as a parent has been maintained for a period in excess of one (1) year.