The Lord hath decreed that the dead should be interred in coffins made of crystal, of hard, resistant stone, or of wood that is both fine and durable, and that graven rings should be placed upon their fingers. He, verily, is the Supreme Ordainer, the One apprised of all.

The inscription on these rings should read, for men: "Unto God belongeth all that is in the heavens and on the earth and whatsoever is between them, and He, in truth, hath knowledge of all things"; and for women: "Unto God belongeth the dominion of the heavens and the earth and whatsoever is between them, and He, in truth, is potent over all things". These are the verses that were revealed aforetime, but lo, the Point of the Bayán now calleth out, exclaiming, "O Best-Beloved of the worlds! Reveal Thou in their stead such words as will waft the fragrance of Thy gracious favours over all mankind. We have announced unto everyone that one single word from Thee excelleth all that hath been sent down in the Bayán. Thou, indeed, hast power to do what pleaseth Thee. Deprive not Thy servants of the overflowing bounties of the ocean of Thy mercy! Thou, in truth, art He Whose grace is infinite." Behold, We have hearkened to His call, and now fulfil His wish. He, verily, is the Best-Beloved, the Answerer of prayers. If the following verse, which hath at this moment been sent down by God, be engraved upon the burial-rings of both men and women, it shall be better for them; We, of a certainty, are the Supreme Ordainer: "I came forth from God, and return unto Him, detached from all save Him, holding fast to His Name, the Merciful, the Compassionate." Thus doth the Lord single out whomsoever He desireth for a bounty from His presence. He is, in very truth, the God of might and power.

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.

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.