Sec. 23. The provisions of 30-4-5-21 shall be effective as to trusts to which such section applies in accordance with the following subsections (a) and (b):
(a) With respect to each such trust first existing on or after January 1, 1970, the provisions of 30-4-5-21 shall be effective from and after the date such trust comes into existence.
(b) With respect to each such trust existing before January 1, 1970, the provisions of 30-4-5-21 shall be effective during taxable years commencing after December 31, 1971.