Know thou that before maturity man liveth from day to day and comprehendeth only such matters as are superficial and outwardly obvious. However, when he cometh of age he understandeth the realities of things and the inner truths. Indeed, in his comprehension, his feelings, his deductions and his discoveries, every day of his life after maturity is equal to a year before it.

Extract from a Tablet of ‘Abdu’l-Bahá, Source

Although the children of Bahá’ís, there is no objection at the present time, for purpose of keeping a correct census, and also ascertaining whether the young people are, sincerely, believers, and willing to do their share in service to the Faith, to asking them to make a declaration of their intention, at the age of fifteen or so.

Shoghi Effendi, Lights of Guidance, p. 154

At the age of 15, they may declare their Faith as a conviction, and be registered as Bahá’í youth,

whether the parents are Bahá’ís or not.

Shoghi Effendi, Lights of Guidance, p. 154

It matters not whether they mature later in one country than in another. The command of Bahá’u’lláh is universal, irrespective of any variance in the age of maturity in different countries and among different peoples.

Shoghi Effendi, Lights of Guidance, p. 233

Regarding children at fifteen a Bahá’í is of age as far as keeping the laws of the Aqdas is concerned -- prayer, fasting, etc. But children under fifteen should certainly observe the Bahá’í Holy Days, and not go to school, if this can be arranged on these nine days.

Shoghi Effendi, Lights of Guidance, p. 154

Regarding the age of fifteen fixed by Bahá’u’lláh; this relates only to purely spiritual functions and obligations and is not related to the degree of administrative capacity which is a totally different thing, and is, for the present, fixed at twenty-one.

Shoghi Effendi, Lights of Guidance, p. 154

The basic principle of the Cause is independent investigation of truth. This applies to us as much as to our children. They should be free to chose for themselves any religion they wish. To promise that they will belong to a certain Faith and not to another is therefore not only contrary to our precepts, but is also a futile promise to give. How can we make the future generation think as we do or follow our dictates. God has made them free. All that we can do is to open their eyes and tell them of what we think to be the truth.

Shoghi Effendi, Lights of Guidance, p. 156

. . . upon attaining the age of fifteen a child becomes spiritually mature and is responsible for stating on his own behalf whether or not he wishes to remain a member of the Bahá’í community. If he does not then reaffirm his faith, he must be treated, administratively, as a non-Bahá’í.

Universal House of Justice, Lights of Guidance, p. 155

...the way in which Bahá’í children should be registered upon reaching the age 15 is within the discretion of each National Spiritual Assembly; there is no objection to using for this purpose the general enrollment card, if such a card is adopted, or a new and separate one specially for Bahá’í children attaining the age of fifteen. It is important, however, that whatever method of enrollments is used or card adopted, it is clear to such children that they had been Bahá’ís up to that time, and that on attaining the age of spiritual maturity they are reaffirming their belief in Bahá’u’lláh.

Universal House of Justice, Lights of Guidance, p. 154

As important as it is for parents to exercise their moral authority in assisting the youth not to make unwise decisions, it is also incumbent on the parents as Bahá’ís to give due consideration to the significance of the spiritual impact of the Faith upon the youth and recognize that the youth must have some latitude to respond to the stirring of their hearts and souls, since they, beginning at the age of 15, must assume serious spiritual obligations and duties and are themselves alone ultimately responsible to God for the progress of their own souls. The capacity for mature thinking on the part of youth differs from one to the other and according to age; some attain this ability earlier than others; for some it is delayed. Parents are generally in a position to judge these matters more accurately than others and must consider them in their attempt to guide the youth in their families, but the parents must strive to do so in such a way as not to stifle their children's sense of spiritual responsibility.

The Universal House of Justice, 1992 Oct 28, Manner of Appealing to Youth

In answer to your letter...concerning the registration of children of Bahá’í parents the Universal House of Justice has instructed us to say that at the present time it prefers to leave the details of such matters to the discretion of each National Spiritual Assembly. One National Assembly, for example, sends a very nice letter to each Bahá’í child in its community on the occasion of its fifteenth birthday (unless, of course, it has reason to doubt that the child in question is a Bahá’í), explaining the meaning of attaining the age of maturity, and extending the good wishes of the Assembly for his or her future services to the Cause. This does not require an active response from every child but does provide each with an opportunity to make his or her position clear if desired. The House of Justice points out that the Assembly must wisely steer a course between seeming to doubt the Faith of a child who has been brought up as a devout Bahá’í on the one hand, and seeming to compel a child to be a member of the Bahá’í community against his will, on the other.

Universal House of Justice, Lights of Guidance, p. 155

The importance of attaining spiritual maturity at the age of fifteen is that it marks that point in life at which the believer takes firmly into his own hands the responsibility for his spiritual destiny.

Universal House of Justice, Messages of the Universal House of Justice, 426.2

Up to the age of 15 years, children are under the direction of their parents.

Universal House of Justice, Lights of Guidance, p. 154