Let us leave aside the question of child being inspired by Sarasvati before he is imbued with Gayatri. The more important thing is that before Kama takes hold of a boy he must be inspired by Gayatri. That is why the age of upanayana is fixed at eight. When one is possessed by Kama one would be dragged away from one's ideal, that of acquiring the power of mantras. Even the power already acquired would be destroyed. That is the upanayana ceremony is performed early so that the boy is helped to become perfect by constant repetition of Gayatri Mantra. After 16, he will not be able to do the same. If he somehow ascends one span spirituality, he will the next moment descend by one cubit. That is why the upanayana samaskara must performed early.
We do not take such samaskaras seriously nowadays. We do things to no purpose, and at the same time we do not have the courage to give up such rites altogether. So we go through them "somehow " for a false sense of satisfaction. Far better it would be, instead, to have the courage to be an atheist. The atheist at least has some convictions, so it seems to me.
If the Gayatri mantra is learned in childhood itself it would be retained like a nail driven into a tender tree. Gayatri imparts in great measure mental strength, lustre and health. It will increase the child's power of concentration, sharpen his intelligence, make him physically strong. Later in life, when he feels the urge of Kama, Gayatri will prevent him from being dragged downward and be a protective shield for his body and intelligence. When one learns to meditate on the Gayatri in childhood itself, it would be a great help, as one grows up, in not wasting one's seed, in acquiring Brahmic lustre and qualities like studiousness, humility, devotion to God and interest in matters of the Self.
Parents nowadays deny their children the opportunity of being afforded such great benefits and for no reason.
A student spends the years of his gurukulavasa in Gayatri-japa; study of the Vedas and the Vedangas, begging for his food, serving his guru, observing various religious vows. When he completes his education thus, he will have become a young man ready for his samavartana. Later he must go to Kasi and, on his return home, take a wife. He is called a "snataka" between his samavartana and his return from his journey to Kasi. Samavartana is equivalent to today's convocation ceremony. In present-day marriages there is a farcical procedure called "Kasi-yatra"
Marriage is one of the forty samskaras.
For a general background, please see here