When the Bhrigus (or Angirases) went to heaven Chyavana was left behind, decrepit and ghostlike. Boys of Sharyata Manava's wandering clan pelted him with clods; he sowed discord so that father fought son. The king questioned his herdsmen, understood, drove out with his daughter Sukanya and gave her in atonement, then moved camp. The Ashvins, wandering the earth as healers, urged her to leave the old man; she would not while he lived. Taught by her husband, she told them they were neither complete nor perfect and would explain once he was young. They sent him into a pool, from which he came out at the age he wished. Chyavana himself then told them: the gods sacrifice in Kurukshetra and exclude you.
On Dhosi Hill near Narnaul people show Chyavana's hermitage and temple and a tank whose water is said to heal skin ailments; pilgrims bathe there on Somvati Amavasya. Local tradition holds that the Ashvins prepared the first chyavanprash here, a herbal paste, to restore the sage.
What is different here:
Youth comes from a medicine (the tonic named after the sage) as well as the bath; ties the story to one hill. Known to us only from Wikipedia's account, not from a temple source.
The hymns know only the result. The Ashvins stripped the skin from old Chyavana like a coat of mail, lengthened his life when all had left him and made him a husband of young women (1.116.10); they restored him to youth (1.117.13, 1.118.6, 7.71.5); they made him, worn with age, young like a chariot able to move again (10.39.4).
What is different here:
No Sukanya, no Sharyati, no pool, no bargain: a bare deed in a list. 'Lord of youthful maidens' (plural) fits no later telling.
Jaiminiya Brahmana (Whitney's translation, quoted by Eggeling)
Pan-IndiaAll traditions
Here the Ashvins ask the sage outright to make them sharers in the Soma, and he asks to be made young first. They draw him into the Sarasvati. He warns his wife that all three will come out looking alike and gives her a sign to know him by. All three come out in the most beautiful of forms, and she recognises him. Young again, he goes to Sharyata and conducts his sacrifice.
What is different here:
The earliest telling with three identical youths and the recognition; the husband arranges the sign himself; the water is the Sarasvati. Only the passage Eggeling quotes was read, not the full Jaiminiya text.
Lomasha tells it at Chyavana's lake. The sage sat so long in the vira posture that an anthill and creepers covered him. Sharyati came with four thousand wives and his one daughter. Sukanya, straying from her maids, saw two bright things in the anthill and pricked them with a thorn. The angry sage stopped the bodily functions of the whole army. Sukanya confessed; Chyavana asked for her as his wife and the king gave her at once. Later the Ashvins saw her after her bath, urged her to choose one of them, and when she refused offered to make her husband young if she would then choose among the three. Chyavana agreed, all three entered the water and came out alike, with shining earrings. She chose her husband. Chyavana promised them Soma before Indra.
What is different here:
Adds the anthill, the pierced eyes and the afflicted army; marriage is the sage's own condition; the riddle of 'incompleteness' is gone, replaced by the choice among three; how she knew is not said.
Sukanya pierces two glowing things in an earthworm's mound and blood flows; the king's men are afflicted and she is given to the irritable sage. Here Chyavana himself asks the visiting Ashvins for youth and promises them a pot of Soma. The three enter a lake of the siddhas and come out alike. Unable to tell them apart, Sukanya takes shelter of the Ashvins, who, pleased with her faithfulness, show her her husband. Her father, visiting, at first rebukes her for sitting with a strange young man.
What is different here:
No proposal by the twins at all; the sage strikes the bargain; the twins themselves point out the husband; adds Sharyati's mistaken rebuke.
Follows the Mahabharata but Chyavana is left blind by the thorn and asks for the princess to care for him; her daily service is described at length. She answers the Ashvins sharply, reminding them the Sun sees all, and threatens a curse. They offer the boon of youth and sight with the test of choosing. Faced with three identical men she prays to the Goddess, who gives her the clear knowledge to recognise Chyavana. The twins then tell him Indra forbade them Soma at Brahma's sacrifice on the golden mountain, and he promises it.
What is different here:
The recognition is the Goddess's gift, answering a prayer; the sage is blind as well as old; the exclusion from Soma is traced to Indra at Brahma's sacrifice.
Sources: The Satapatha-Brahmana, tr. Julius Eggeling (Sacred Books of the East 12, 41); Wikipedia: Dhosi Hill; Wikipedia: Chyavana; Hymns of the Rigveda, tr. R. T. H. Griffith (1896), hymns read for the Vedic layer; The Mahabharata, tr. K. M. Ganguli (1883-96), sections read for the Vedic layer; Srimad-Bhagavatam (Bhaktivedanta Vedabase); The Srimad Devi Bhagawatam, tr. Swami Vijnanananda (1921-22), Book 7 chapters 2-7