The Kitáb-i-Aqdas - The Most Holy Book
Index term: Prayer for the Dead and burial rings, under for performance of religious duties; Maturity, age of
n10.
the Prayer for the Dead
The Prayer for the Dead (see Some Texts Supplementary to the Kitáb-i-Aqdas) is the only Bahá'í obligatory prayer which is to be recited in congregation; it is to be recited by one believer while all present stand in silence (see note 19). Bahá'u'lláh has clarified that the Prayer for the Dead is required only when the deceased is an adult (Q&A 70), that the recital should precede the interment of the deceased, and that there is no requirement to face the Qiblih when saying this prayer (Q&A 85).
Further details concerning the Prayer for the Dead are summarized in the Synopsis and Codification, section IV.A.13.-14.
n149.
The Lord hath decreed that the dead should be interred in coffins
In the Bayán, the Báb prescribed that the deceased should be interred in a coffin made of crystal or polished stone. Shoghi Effendi, in a letter written on his behalf, explained that the significance of this provision was to show respect for the human body which "was once exalted by the immortal soul of man".
In brief, the Bahá'í law for the burial of the dead states that it is forbidden to carry the body for more than one hour's journey from the place of death; that the body should be wrapped in a shroud of silk or cotton, and on its finger should be placed a ring bearing the inscription "I came forth from God, and return unto Him, detached from all save Him, holding fast to His Name, the Merciful, the Compassionate"; and that the coffin should be of crystal, stone or hard fine wood. A specific Prayer for the Dead (see note 10) is ordained, to be said before interment. As affirmed by 'Abdu'l-Bahá and the Guardian, this law precludes cremation of the dead. The formal prayer and the ring are meant to be used for those who have attained the age of maturity, i.e. 15 years of age (Q&A 70).
With regard to the material from which the coffin is to be made, the spirit of the law is that coffins should be of as durable a material as possible. Hence, the Universal House of Justice has explained that, in addition to the materials specified in the Aqdas, there is no objection to using the hardest wood available or concrete for the casket. For the present, the Bahá'ís are left free to make their own choices in this matter.
q70.
 
Question: Is the use of the burial ring enjoined exclusively for adults, or is it for minors as well?
Answer: It is for adults only. The Prayer for the Dead is likewise for adults.