If a person who is entitled to compensation under this chapter is mentally incompetent or a minor, the provisions of subsection (a) shall not be applicable so long as such person has no guardian or other authorized representative, but shall be applicable in the case of a person who is mentally incompetent or a minor from the date of appointment of such guardian or other representative, or in the case of a minor, if no guardian is appointed before he becomes of age, from the date he becomes of age.