to be muslim.

to have reached puberty.
however, with young children it was the practice of the prophet's companions to encourage them to fast in order for them to get used to it at an early age. [3]

to be sane.
‘alee ibn abi talib and ‘a'ishah both reported that prophet muhammad (peace be upon him) said, "the pen is raised from (the book) of three (individuals): the sleeper until he awakes, the child until he reaches puberty, and the insane until he regains his sanity." [4]

to be physically able.

to be a resident, not a traveler.
a menstruating woman does not pray or fast. prayers need not be made up, but fasting must be made up later when her menses are finished. allah has permitted muslims who are sick or physically unable to fast or are traveling during ramadan to break their fasts and make up the missed days of ramadan. allah says in the quran:

"...and whoever is ill or journeying, for him is the like number of other days. allah desires for you ease and does not desire for you hardship; so you should complete the number and magnify allah for his having guided you, that perhaps you may give thanks." 

[noble quran 2:185]