Aitareya is one of the earliest Upanishads and is attributed to sage Mahidasa Aitareya. It consists of five sections, viz. sections 21 to 25. These sections form the 4th, 5th, and 6th chapters of the 2nd part of the Rigveda Aranyaka.

For

the young student the मन्त्र Samhita verses.
the householder the ब्राह्मण Brahmana verses.
the retired the आरण्यक Aranyaka verses.
the sannyasi the उपनिषद् Upanishad verses.

Young Student = one who is yet being cared for by parents.

Householder = one who is in the thick of earning, raising a family, or actively engaged in society.

Retired = one who is out of the grind and living on savings.

Sannyasi = one who is free in mind and light at heart.

These four phases are not separated in time or by age. These may occur in any order in a man's life, sometimes concurrent, sometimes distinct. One may have the Sannyasa experience earlier and get thrown into the Householder struggle later. It happens. The key point to understand is that these are states of the mind, heart, intellect and emotion, and one's relationships in society.

Using simple yet elegant words, the Rishi narrates the story of creation from the Big Bang onward. The complete paraphernalia for its functioning is described, also the biology of man's birth. An incisive statement is made that points to the undeniable fact "Opposite Values are Complementary in Nature".

Its mahavakya is praj ānaṃ brahma, i.e. Consciousness = the Divine = the Supreme.

सोऽस्यायमात्मा पुण्येभ्यः कर्मभ्यः प्रतिधीयते । अथास्यायमितर आत्मा कृतकृत्यो वयोगतः प्रैति । स इतः प्रयन्नेव पुनर्जायते तदस्य तृतीयं जन्म ॥ २.१.४

so'syāyamātmā puṇyebhyaḥ karmabhyaḥ pratidhīyate । athāsyāyamitara ātmā kṛtakṛtyo vayogataḥ praiti । sa itaḥ prayanneva punarjāyate tadasya tṛtīyaṃ janma ॥ 2.1.4

The soul travels through bodies. It also travels through traits and virtues. Since a Soul gets known and remembered in diverse ways

Through physical lineage.
Through propagation of ideals.
Through celebration of festivals and anniversaries.

Hence that is known as the Undying Birth.

Giving the example of sage Vamadeva who attained enlightenment, a devotee is inspired and empowered to embark on an inward journey...

Aitareya Upanishad - häftad, Sanskrit, 2020