Upanayana is a "samskaara" (the cleansing, sacramental, training, refining) process to enable the learning for a child. Over a period of time it has been limited to the boychild and the girl child's upanayana supposedly happened during the marriage.
Upanayana, as defined by sage Bharadhwaja is "उपनयाख्यं यत्कर्म विद्यार्थं तदुदीरितम्", "उपसमीपे आचार्यादीनां वटॊः नीतिः नयनं प्रपणं उपनयनं", i.e. The process of sending the student to the proximity of a great practitioner, a guru, is called upanayanam. Or, "आचार्यादीनं समीपे वटुः नीयते येन संस्कारॆण कर्मण तत् संस्कारकर्म उपनयनम्", that samskaara that takes the student closer to an aachaarya is said to be upanayana. The aachaarya instructs the pupil on a variety of subjects but the initiation happens via the gayatri manta, the essence of all the vedic injunctions. Sage Katyayana says "गायत्र्या ब्राह्मणं उपनयीत | उपनयनं विद्यार्थस्य शृतितः संस्कारः". i.e. Upanayanam is a samskaara that is done via the sacred texts of the vedas.
Manu defines this process as "brahmajanma" as below:
Such a beautiful way to describe the "rebirth" of the student, where knowledge is said to create that new individual; where "ray of light" (gayatri, a verse in praise of savitru devata) becomes the cause of all learning and the aacharya seeds this in the student, thus becoming the father.
aapasthambha dharma sootra says:
This is not to discount the role of parent but to highlight the greatness of knowledge and education.
If rightly done this samskaara sows the right seeds in the prospective student. There are three key expected outcomes of the upanayana samskaara:
1) shraddhaa - the inclination and motivation to learn
2) mEdha - the ability to grasp and retain the learning
3) prajnA - ability to connect the learning to practice; to view the past, the present and the future in a logical, reasoned, rational way.
So the whole reason for this samskaara is to enable an individual to be educated, to learn, and to mature. It is the initiation for the student to contribute positively to the society in the end.
There is a second reason why this samskaara is important. This samskaaraa can cleanse some of the "dOshAs" of the previous janma - the current bodily birth and also that of the previous birth, by the grace of all the deities that are invited, worshiped and sought blessings from. This process is ideally done at the age 7, because that is a critical age for learning and connecting dots. This samskaara is prescribed for all varNas even though the curricula may vary.
There is a beautiful sanskrit saying: "सा विद्या या विमुक्तयॆ", that is education which is liberating. Manu warns that if this samskaara is not done on time then the critical learning time will have past.
Manu infact prescribes age 5, 6, and 8 for the three varnas to initiate into the process:
ब्रह्मवर्चसकामस्य कार्यं विप्रस्य पंचमे ।
राज्ञोबलार्थीनः शष्ठॆ वैश्यस्यचार्थिनॊऽष्टमॆ ॥
For the attainment of brahma-varchas a 'brahmaNa' should perform upakarma when the child is 5 year old. For the attainment of strength, a 'kshatriya' should perform when the child is 6. And, for the attainment of wealth and business a 'vaishya' should perform it when the child is 8.
In essence, early education is the key.
The primary responsibility of a parent is to find the right place, right space, right time, and right guru to perform this important samskaara. Sri Vyaasadeva prescribes a special prayer that the parent is supposed to make regarding the one who performs the samskaara, that he be:
One should pray and desire for the performer (the guru, who leads the samskaara) has the following attributes: complete conviction of the vedic injunctions (that he is competent), is rightful and knows the dharma well, has a good background (endowed with good samskaara himself), is well learned in the vedas (modest, conversant with sacred scriptures), pure ('clean character'), is knowledgeable of his vedic branch, is not lazy (is energetic), and is a strong theologian.
I will not get into discussion about when, at what specific time of the year, and how this samskaara should be performed. That is the ritualistic side, which can change from one custom to another. In the next part I will try to discuss the specific details or the significance of janivaara and so on.
Reference: samskaaramahOdadhiH, Dr. SacChaMdaM AnantanarasimhAchAr