Dear Baha'i friend,
The Universal House of Justice received your very thoughtful letter and instructs us to convey the following answer to you.
While some opportunities for service in the Administrative Order are clearly reserved for those who are over twenty-one years of age, 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. At age fifteen, the individual has the privilege of affirming, in his own name, his faith in Baha'u'llah. For while the children of Baha'i parents are considered to be Baha'is, they do not automatically inherit the Faith of their parents. Therefore, when they come of age, they must, of their own volition, express their belief.
Having reached the age of fifteen, Baha'i youth are personally responsible for certain spiritual activities such as observing the obligation of daily prayer, keeping the Fast, and they are invited to participate in Baha'i youth activities. The significance of the age of maturity, however, goes far beyond the fulfilment of responsibilities. The following extract from a Tablet of 'Abdu'l-Baha links the attainment of maturity with the deepening of one's understanding and comprehension of the realities of life, and the enhancement of one's very capacity for understanding:
“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.”
The signing of a card is simply the means by which the individual indicates his desire to be registered as a Baha'i youth, as a member of the Baha'i community, and it enables the National Spiritual Assembly to keep an accurate record of the membership of the community. ...
The Universal House of Justice assures you that it will offer prayers at the Holy Shrines that you will strive to become an active and enlightened servant of the Cause of God.
With loving Baha'i greetings,
Department of the Secretariat
(‘Messages from the Universal House of Justice 1963-1986’)