Which is considered the oldest Upanishad?

The Brihadaranyaka Upanishad is generally considered the oldest Upanishad, along with one or two other early prose Upanishads.

About the Brihadaranyaka Upanishad:

• "Brihad" means "great" and "aranyaka" means "of the forest"; it is the "great forest text".
• It is part of the Shatapatha Brahmana of the Shukla Yajurveda.
• It is also the longest of the principal Upanishads.
• It was composed around the 8th or 7th century BCE, according to most scholars.

Its contents:

• It contains the teachings of the sage Yajnavalkya, one of the greatest thinkers of the Upanishads.
• It includes the famous debates at the court of King Janaka of Videha, in which Yajnavalkya answered questions from other sages, including the woman philosopher Gargi Vachaknavi.
• It has the dialogue between Yajnavalkya and his wife Maitreyi on the nature of the self and immortality.
• It contains the famous prayer "Asato ma sadgamaya, Tamaso ma jyotirgamaya, Mrityor ma amritam gamaya" ("Lead me from the unreal to the real, from darkness to light, from death to immortality").
• It contains the Mahavakya "Aham Brahmasmi" ("I am Brahman").
• It describes the self (atman) as "neti, neti" ("not this, not this"), meaning that it cannot be described by any quality.

Classification of Upanishads by age:

• Early prose Upanishads: Brihadaranyaka and others.
• Later verse Upanishads: Katha, Isha, Shvetashvatara, Mundaka.
• Later prose Upanishads: Prashna, Mandukya.

Why not the other options?

• ✗The Isha Upanishad is a short verse Upanishad, the first in the traditional list of 108.
• ✗The Mandukya Upanishad is the shortest Upanishad.
• ✗The Shvetashvatara Upanishad is a later verse Upanishad with theistic ideas.