Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 7 · Samskara Muhurtas
संस्कारमुहूर्ताः
आधानादष्टमे वर्षे जन्मतो वाग्रजन्मनाम् ।राज्ञामेकादशे मौंजीबंधनं द्वादशे विशाम् ॥ ३४८ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 348.
For Brahmins, tie the munja-grass girdle in the 8th year from conception or from birth. For kings it is the 11th year. For Vaishyas it is the 12th.
Simple meaning: The sacred thread is tied in the 8th year for Brahmins, the 11th for kings and the 12th for Vaishyas.
जन्मतः पंचमे वर्षे वेदशास्त्रविशारदः ।
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.