Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 7 · Samskara Muhurtas
संस्कारमुहूर्ताः
जन्मतः पंचमे वर्षे वेदशास्त्रविशारदः ।उपवीती यतः श्रीमान्कार्यं तत्रोपनायनम् ॥ ३४९ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 349.
A boy given the sacred thread in his 5th year from birth becomes skilled in the Veda and the sciences. He becomes prosperous. So the thread rite should be done then.
Simple meaning: The sacred thread in the 5th year can make a boy learned and prosperous.
बालस्य बलहीनोऽपि सितो जीवः शुभप्रदः ।
Venus and Jupiter help a boy even when weak. Keep to the years given for the rite.
दृश्यमानगुरौ शुक्रे शाखेशे चोत्तरायणे ।
Do the thread rite when Jupiter, Venus and the lord of one's Veda are visible, in the Sun's northward course.
शरद्ग्रीष्मवसंतेषु व्युत्क्रमात्तु द्विजन्मनाम् ।
Autumn, summer and spring, in reverse order for the three twice-born groups, are best. Five months from Magha are ordinary.