There are only three conditions for fasting to be Fardh:

[1] Islam
[2] Sanity
[3] Bulūgh (i.e. being mature)

When a person is Muslim, sane and mature, he/she has to fast. The only other condition is that he/she should not be a mā’zūr (a person who has a genuine excuse such as severe illness where there is no possibility of recovering).

Extracted from A Gift for Ramadhan, Page 48.

A Gift For Ramadhan – New 3rd Revised Edition