1. Who must fast in Ramadan?

a. Every accountable Muslim is obligated to fast the month of Ramadan. To be of full age, the age of puberty and discretion, which is normally about fourteen. Children under this age should be encouraged to start this good practice on easy levels, so when they reach the age of puberty they will be mentally and physically prepared to observe the fasting. 
b. For one to be obligated to fast, one must be of sound mind/sane.

2. Exemption from fasting
a. Fasting is not obligatory on the insane person.
b. Fasting is not obligatory on a person whose body cannot tolerate fasting, either due to old age or severe illness.
c. The one who is travelling a walking distance of two or more days (about 80 miles or 130 kilometres) is permitted to break their fast, provided his reason for travelling is not sinful. However, one must make up the missed days.
d. Fasting is not obligatory on the menstruating woman, or the woman who has postpartum bleeding. The pregnant woman is permitted to break her fast if she fears harm may come to her or her baby from fasting. This includes the breastfeeding woman as well.

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References:
http://yushaevans.com
http://askaquestionto.us
http://quran.com
http://www.myot.org.au
http://www.fastramadan.com
http://sunnah.org
http://www.sunniforum.com