Permanent residents of this unit must be at least 55 years of age, except that the spouse or an immediate member of the family, other than a child of said permanent resident, or a live-in domestic, companion or nurse, may be a permanent resident regardless of his or her age. A maximum of one child, age 18 or older, may also reside as a permanent resident with his or her parent or parents. In no event, however, shall there be more than three permanent residents in one senior citizen residential dwelling unit.