In the life of a Hindu boy of the three communities, namely Brahmins, Kshatriyas, and Vysyas, Upanayana marks the beginning of Brahmacharya Ashrama which is set apart for the study of Vedas; this stage is said to be the second birth for him. Mere wearing of a sacred thread initiated on the Upanayana day does not make him a real Brahmacharin. It is rather the study of Vedas, learning and chanting Vedic mantras including Gayathri and Sandhyavandhana which will qualify him to be a true Brahmacharin. In modern education, there is no place for Hindu Scriptures and very few study Vedas after Upanayana. The least that is expected to be done after this ceremony is to recite the Gayathri mantra and do Sandhyavandhana. The ceremony is not given much importance or practiced with religious fervour by these communities. No attempt is made by the parents or the religious preceptors to educate the youngsters on the importance of this ceremony and its spiritual values. The ceremonies are done in a routine manner and the wards take this sacred religious ritual with least seriousness. The net result is complete ignorance and degeneration of values.
There possibly cannot be a better authority to enlighten us on Dharma Sastras and Vedas than the Maha Periyaval of Kanchi Kamakoti Peetham . The best explanation and thoughts on Upanayana are available in the book on Hindu Dharma published by the Bharatiya Vidya Bhavan which contains the English translations of lectures given by him. As requested by the readers, I have given relevant excerpts below from the book. These are not exhaustive and readers are advised to go through the original to get a deeper understanding of the subject.
(Excerpts from pages 531-559 of the book on Hindu Dharma, published by the Bharatiya Vidya Bhavan, which contains the English translations of lectures given by the Maha Periyaval of Kanchi Kamakoti Peetham.)
"The upanayana of a boy is performed when he is old enough to understand things and chant mantras. During this ceremony, he is asked to go begging for alms. When he starts learning at the age of five, he will have basic knowledge of Sanskrit by the time he is eight years old, the age fixed for the Upanayana Samskara. The world will stand to gain if eight year old children wear sacred thread, have sufficient knowledge of Sanskrit and chant the Gayatri mantra."
"Upanayana means to take or lead the child near the Guru. Upanayana is initiation into the brahmacharyasrama and is the purvanga of student bachelor-hood. For students of krsna-yajurveda, there are four "vratas" between purvanga called Upanayana and uttaranga called samavartana. These are prajapatya, saumya, agneya and vaisvadeva. For Rig-veda, it is different; each Veda ha sits own Vratas. Everybody must learn his own veda and other subjects in addition. When we perform upakarma, we must start learning a new part of Vedas. Later at the time of utsarjana, it must be discontinued and the study of vedanga taken up. The vedas must be studied during the six months of Sravana to Taisya. The next six months must be devoted to vedangas."
"To master the mantras, the student must strictly observe the rules pertaining to brahmacharya and to the particular part of the vedas that is being studied. The importance of the upanayana ceremony lies in this: it makes a person fit to receive instruction in the vedas and spread their divine power through out the world. Parents must realise this fact and perform their son's upanayana at the right time."
"The upanayana samskaras must not be postponed on any pretext whatsoever. Some wrong beliefs which have no sastric validity are: 1. The Upanayana of a son must not be performed if he has an elder sister yet to be married. 2. Three brahmacharins must not stay together in a family at the same time. These customs have originated as a matter of convenience or for some sentimental reasons. There is no valid excuse for failure to perform the upanayana at the right time."
"A brahmin child's upanayana must be performed when he is eight years old from conception, that is when he is seven years and two months old from birth. A kshatriya's is to be performed at the age of twelve. Krsna Paramatman who belonged to the clan of Yadavas was invested with the sacred thread at that age. The corresponding age for a Vaisya is sixteen. According to sastras, the lower limit for a brahmin youngsters is eight years and the upper limit sixteen which means a grace of eight years."
"Uttarayana is the right period to perform upanayana-from the Tamil month of Thai to Ani when the sun journeys northwards. Spring, (Chittirai, Vaikasi) is particularly auspicious. The month of Masi (mid-Feb to mid March) is specially favoured. Unlike marriages, Upanayana should never be permitted in Dakshinayana."
"It is the duty of parents to make sure that, after they are invested with the sacred thread, their sons chant the a Gayathri every day without fail along with sandhyavandana. If Gayathri mantra is learnt in childhood itself it would be retained like a nail driven into a tender tree. Gayathri imparts a great measure of mental strength, luster and health. It will increase the childs power of concentration, sharpen his intelligence, make him physically strong. Later in life, when he feels the urge of kama. Gayathri will prevent him from being dragged downwards and be a protective shield for his body and intelligence. Gayathri japa is essential to all rites performed according to the sastras. At least on Sundays, all those who wear sacred thread must do Gayathri japa a thousand times. Even in times of misfortune the Gayathri mantra must be muttered at least ten times at dawn, midday and dusk. These are hours of tranquility. Gayathri contains in itself the spirit and energy of all the vedic mantras. Indeed it imparts power to other mantras. Gayathri is the hypnotic means of liberating ourselves from worldly existence as well as controlling desire and realising the goal of our birth."