The Kitáb-i-Aqdas - The Most Holy Book
Index term: age of maturity for, under Fasting
k10.
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.
n13.
We have commanded you to pray and fast from the beginning of maturity
Bahá’u’lláh defines the "age of maturity with respect to religious duties" as "fifteen for both men and women" (Q&A 20). For details of the period of fasting, see note 25.
n25.
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).