Az-Zakah is obligatory on the wealth of each of them, if the person is a free Muslim who has complete ownership of his wealth. This is due to what Ad-Daraqutni narrated that was reported as a statement of the Prophet (Sallallaahu alayhi wasallam): "Whoever is made the guardian over the wealth of an orphan, let him do business with it and he should not leave it to be devoured by chrity i.e, Az-Zakah)." [Ad-Daraqutni 2:109 no. 1951.]
This is due to what Malik narrated in Al-Muwatta' from 'Abdur-Rahman bin Al-Qasim from his father, that he said:
"A'ishah used to keep me and my brother as two orphans in her appartment, and she used to pay Az-Zakah from our wealth."
The view that Az-Zakah is obligatory upon the wealth of each of them (the orphan and the insane person)was held by 'Ali, Ibn 'Umar, Jabir, A'shah and Al-Hasan bin 'Ali. Ibn Al-Mundhir related this from them.