No scriptures describe the life story of Mahidasa Aitareya.. But he is briefly mentioned in three places in Hindu scripture. First of all, in this chapter of the Aitareya Aranyaka of the Rig Veda:
Was it water really? Was it water? Yes, all this was water indeed. This (water) was the root (cause), that (the world) was the shoot (effect). He (the person) is the father, they (earth, fire, &c.) are the sons. Whatever there is belonging to the son, belongs to the father; whatever there is belonging to the father, belongs to the son. This was intended. Mahidâsa Aitareya, who knew this, said: 'I know myself (reaching) as far as the gods, and I know the gods (reaching) as far as me. For these gods receive their gifts from hence, and are supported from hence.'
And he's mentioned in another chapter of the Aitareya Aranyaka of the Rig Veda:
This (nishkevalya-sastra) becomes perfect as a thousand of Brihatîs. It is glory (the glorious Brahman, not the absolute Brahman), it is Indra. Indra is the lord of all beings. He who thus knows Indra as the lord of all beings, departs from this world by loosening the bonds of life - so said Mahidâsa Aitareya. Having departed he becomes Indra (or Hiranyagarbha) and shines in those worlds.
Finally, he's mentioned in this chapter of the Chandogya Upanishad:
Man is sacrifice. His (first) twenty-four years are the morning-libation... If anything ails him in that (early) age, let him say: 'Ye Prânas, ye Vasus, extend this my morning-libation unto the midday-libation, that I, the sacrificer, may not perish in the midst of the Prânas or Vasus.' Thus he recovers from his illness, and becomes whole. The next forty-four years are the midday-libation.... If anything ails him in that (second) age, let him say: 'Ye Prânas, ye Rudras, extend this my midday-libation unto the third libation, that I, the sacrificer, may not perish in the midst of the Prânas or Rudras.' Thus he recovers from his illness, and becomes whole. The next forty-eight years are the third libation... If anything ails him in that (third) age, let him say: 'Ye Prânas, ye Âdityas, extend this my third libation unto the full age, that I, the sacrificer, may not perish in the midst of the Prânas or Âdityas.' Thus he recovers from his illness, and becomes whole. Mahidâsa Aitareya, who knew this, said "Why dost thou afflict me, as I shall not die by it?" He lived a hundred and sixteen years.He, too, who knows this lives on to a hundred and sixteen years.
Now as I said the life story of Mahidasa Airareya isn't described in any scripture, but it is described in Sayana's commentary on the Aitreya Brahmana of the Rig Veda, quoted in this book:
There was once a great sage who had many wives. Among them was one named Itara. This Itara had a son; the boy was called Mahidasa. It is said in the part of the Aranyaka: "Mahidasa Aitareya spoke, indeed, such." The father of this Mahidasa had greater love towards his son from another wife, than that towards Mahidasa. Once, in a sacrificial assembly he demonstrated scorn towards Mahidasa, because he took the other son on his lap. Thereupon his mother, Itara, when she saw the afflicted face of Mahidasa, thought of their family goddess, Earth. Then as it happened the goddess Earth appeared in her celestial form before the sacrificial assembly, presented Mahidasa a heavenly thronelike chair and seated him in it. Then she proclaimed his superiority in knowledge over all other boys and conferred on him as a gift the mental vision of the present Brahmana,. Through her mercy, the Brahmana consisting of forty chapters was revealed through the mind of Mahidasa. The Brahmana begins with "Agnir vai devanam avamah" and ends with "Strinute Strinute". Further, also the portion of the Brahmana beginning with "atha Mahavratam" and ending with "acharya, acharyah" was revealed to him for the vow of life in the forest.
Also, earlier than Sayana, the Dvaita philosopher Madhvacharya said in his commentary on the Aitareya Upanishad that Mahidasa Aitreya was an incarnation of Vishnu, as described in this book:
[Madhvacharya] state[s] the original speaker of this Upanishad to be Mahidasa, an incarnation of Narayana, proceeding from Visala, son of Abja. He adds, that on the sudden appearance of this deity at a solemn celebration, the whole assembly of gods and priests fainted, but at the intercession of Brahma, they were revived; and after making their obeisance, they were instructed in holy science. This Avatara was called Mahidasa, because those venerable personages (Mahin) declared themselves his slaves (dasa).