One of the primary duties of Brahman, Kshatriya and Vaisya to wear Upaveeda,Jaanu, பூணூல்.

Upaveeda is worn after Upa Nayana.

One is called Dwija,who is born for the second time.

When an individual is born,he is clouded with Ignorance.

With no awareness of Reality.

After. Initiated into Brahmacharya,

the first station in Life,the others being Married (Gruhastha),Semi retirement from worldly affairs(Vanaprastha) and Renunciation at things Material,Sanyasa,one is empowered to enter the path of Spirituality.

Upa Nayana means ,’being taken near to Brahman,The Reality.

A Perceptor,Guru takes the Sishya, Disciple near Brahman.

Another explanation is that is after Upa Nayana,the second eye,as it were,is opened towards Reality.

Upa Nayana is to be performed before

The age of,

Eight for Brahmins,

Twelve for Kshatriya,

Sixteen for Vaisya.

Upaveeda is worn across the left shoulder while performing Auspicious Duties,

Across the Right shoulder, while performing Inauspicious duties and Pitru Karma (Pracheena Veedhi)and as

Niveedhi, around the neck like a garland when we perform our worldly duties.

Reference. Sandhya Vandana by Anna Subrahmanya Iyer,RK Mutt Publications.