We have commanded you to pray and fast from the beginning of maturity; this is ordained by God, your Lord and the Lord of your forefathers. He hath exempted from this those who are weak from illness or age, as a bounty from His Presence, and He is the Forgiving, the Generous. God hath granted you leave to prostrate yourselves on any surface that is clean, for We have removed in this regard the limitation that had been laid down in the Book; God, indeed, hath knowledge of that whereof ye know naught. Let him that findeth no water for ablution repeat five times the words "In the Name of God, the Most Pure, the Most Pure", and then proceed to his devotions. Such is the command of the Lord of all worlds. In regions where the days and nights grow long, let times of prayer be gauged by clocks and other instruments that mark the passage of the hours. He, verily, is the Expounder, the Wise.

He hath exempted from this those who are weak from illness or age
The exemption of those who are weak due to illness or advanced age from offering the Obligatory Prayers and from fasting is explained in Questions and Answers. Bahá’u’lláh indicates that in “time of ill-health it is not permissible to observe these obligations” (Q&A 93). He defines old age, in this context, as being from seventy (Q&A 74). In answer to a question, Shoghi Effendi has clarified that people who attain the age of seventy are exempt, whether or not they are weak.
Exemption from fasting is also granted to the other specific categories of people listed in the Synopsis and Codification, section IV.B.5. See notes 20, 30 and 31 for additional discussion.

We have enjoined upon you fasting during a brief period
Fasting and obligatory prayer constitute the two pillars that sustain the revealed Law of God. Bahá’u’lláh in one of His Tablets affirms that He has revealed the laws of obligatory prayer and fasting so that through them the believers may draw nigh unto God.
Shoghi Effendi indicates that the fasting period, which involves complete abstention from food and drink from sunrise till sunset, is

. . . essentially a period of meditation and prayer, of spiritual recuperation, during which the believer must strive to make the necessary readjustments in his inner life, and to refresh and reinvigorate the spiritual forces latent in his soul. Its significance and purpose are, therefore, fundamentally spiritual in character. Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires.

Fasting is enjoined on all the believers once they attain the age of 15 and until they reach the age of 70 years.
A summary of the detailed provisions concerning the law of fasting and of the exemptions granted to certain categories of people is contained in the Synopsis and Codification, section IV.B.1.-6. For a discussion of the exemptions from fasting see notes 14, 20, 30 and 31.
The nineteen-day period of fasting coincides with the Bahá’í­ month of 'Alá', usually 2-20 March, immediately after the termination of the Intercalary Days (see notes 27 and 147), and is followed by the feast of Naw-Rúz (see note 26).

The traveller, the ailing, those who are with child or giving suck, are not bound by the Fast; they have been exempted by God as a token of His grace.
Exemption from fasting is granted to those who are ill or of advanced age (see note 14), women in their courses (see note 20), travellers (see note 30) and to women who are pregnant and those who are nursing. This exemption is also extended to people who are engaged in heavy labour, who, at the same time, are advised “to show respect to the law of God and for the exalted station of the Fast” by eating “with frugality and in private” (Q&A 76). Shoghi Effendi has indicated that the types of work which would exempt people from the Fast will be defined by the Universal House of Justice.

Question: Concerning the definition of old age.
Answer: To the Arabs it denoteth the furthest extremity of old age, but for the people of Bahá it is from the age of seventy.