Effective date of exemption

An exemption received by an individual pursuant to this subsection shall be effective for the first taxable year for which he has net earnings from self-employment (computed without regard to subsections (c)(4) and (c)(5)) of $400 or more, any part of which was derived from the performance of service described in subsection (c)(4) or (c)(5), and for all succeeding taxable years. An exemption received pursuant to this subsection shall be irrevocable.