Subject to any provision in the certificate of incorporation authorized by paragraph (b) of section 620 (Agreements as to voting; provision in certificate of incorporation as to control of directors) or by paragraph (b) of section 715 (Officers), the business of a corporation shall be managed under the direction of its board of directors, each of whom shall be at least eighteen years of age. The certificate of incorporation or the by-laws may prescribe other qualifications for directors.