For the purposes of custody, the rights of a person who is mentally or physically incapable of self-support upon reaching his majority shall be the same as a minor child for so long as he remains mentally or physically incapable of self-support. (1967, c. 1153, s. 2; 1971, c. 218, s. 3; 1973, c. 476, s. 133; 1979, c. 838, s. 29; 1989, c. 210.)