Those who should fast and those exempted

Sayyid Saabiq said in Fiqhus-sunnah: “ All the Islamic scholars agreed that fasting is only compulsory on a Muslim who is sane, who has reached the age of puberty, who is healthy and who is resident (not on journey). They also agreed that it is only compulsory on a woman who is pure from menses […]

Sayyid Saabiq said in Fiqhus-sunnah: “ All the Islamic scholars agreed that fasting is only compulsory on a Muslim who is sane, who has reached the age of puberty, who is healthy and who is resident (not on journey). They also agreed that it is only compulsory on a woman who is pure from menses and postal natal blood.”
   As for the evidence that says fasting is not compulsory on insane Muslim and Muslim who has not reached puberty, it is the saying of the Prophet (PBUH): “The pen has been raised (sin will not be recorded) in respect of three (categories of people): on a mad man/woman until he/she fully recovers, on a sleeping person until he wakes up and on immature boy/girl until he/she reached puberty.{Sahih-Jam’i}.
But it is recommended for a parent or guardian to encourage his child, who has not reached puberty, to fast if he is able to in order to make him familiar with fasting, and in imitating the practice of the companions of the prophet with their children. Rubayyi’u bint Mu’awiz said: The prophet (Allah’s blessings and peace be upon him) sent message to the localities of Ansaru in the morning of ‘Ashura, saying: ‘whoever wakes up not fasting (because he did not know that there is fasting)  should continue to fast the day, and whoever wakes up fasting should complete the fast’. She (Rubayyi’u) then said: we used to fast ‘Ashura afterwards and we made our small children to fast it, and we used to make toys for them to keep them busy. So when one of them cried because he wanted food, we would then give him/her the toy until the time of iftaar reaches. {Bukhari and Muslim}.
As for fasting not compulsory on sick Muslim and Muslim on a journey, the evidence is the saying of Allah in chapter two verse 183 of the holy Qur’an: Whoever is ill or on a journey should make up (the missed days ) on another days.
But if a sick Muslim and a traveler can fast, the fasting would be accepted because the permissibility for them not to fast is only an allowance. So if they choose to do the main thing by fasting it is best as long as they do not experience difficulty in doing so.
So if a sick Muslim and traveler experience difficulty in their fasting, it is best for them to break the fast. But if they do not experience difficulty, to fast is better for them. Abu Sa’eed Al-Khudriy (AS) said: We were on expedition with the Messenger of Allah (PBUH) in the month of Ramadan. And there were people among us who fasted and there were people who did not fast, those who fasted did not look down on those who did not fast and those who did not fast did not look down on those who fasted. They (we) held that those who have the energy it was best for them to fast and they equally held that those who were weak to break the fast for them was best. {Saheehu-tTirmidhi}.
As for the evidence that stipulates that women must not fast during their menses and postal natal bleeding, it is the hadeeth reported by Abuu Sa’ed (RA) that the Prophet (PBUH) said: Is it not  that when ( a woman) menstruates she does not observe salah and she does not fast? And that is the defect in her religion. {Bukhary}.
If a woman who experiences menses or postal natal bleeding fasts, the fast will not be accepted because from the conditions of fasting is that a woman must be pure from menses and postal natal bleeding.
 But it is compulsory on her to make the missed days up after the month of Ramadaan. Nana ‘Aisha (RA) said: “we menstruated during the life time of the Messenger (PBUH), we were commanded to make up the missed days of fasting but we were not commanded to pay back the salah we missed. {Tirmidhi, authenticated by Imam Al-Bani}.