Land and improvements held by a church, religious society, charitable corporation, or nonprofit organization principally devoted to housing of elderly persons and not held for investment and which is constructed pursuant to the provisions of § 231 of the National Housing Act, as presently in effect and set forth in 12 U.S.C. § 1715v and which is regulated by the Federal Housing Commissioner pursuant to such section and pursuant to the applicable federal regulations presently in effect adopted by the Federal Housing Commissioner, shall not be liable to taxation and assessment for public purposes by any county or other political subdivision of this State; provided, however, that no less than 75% of the dwelling units in the property shall be rented and occupied at all times by elderly persons as defined in § 8152 of this title and the entire property shall be operated on a nonprofit basis.