The following excerpt is from the Life of Saint Paisios the Great written by his disciple Saint John the Dwarf in the early 5th century, and is one of the earliest references to the Seven Youths of Ephesus. They are mentioned in the context of Saint John describing how it was possible by God's grace to preserve Saint Paisios alive when he didn't eat for seventy consecutive days, just as the Seven Youths of Ephesus were preserved alive while sleeping for many decades.
"After communion with the Divine Mysteries of the Body and Blood of Christ, Venerable Paisios sometimes remained without bodily food for seventy days; and there is nothing surprising in this, for divine grace has an indescribable power and therefore could support life in him better than refreshment with corruptible food. For those who live according to the carnal nature, the body, in order not to become exhausted, require to strengthen it with nutrition, and those who, like the incorporeal, have succeeded in a higher natural life, the divine power in abundance gives this grace, which human nature obeys, and lives no longer on bodily food, but on spiritual food. The Almighty Creator, our God, knows how to preserve the lives of seven sleeping youths on earth for three hundred years and even more. And indeed He preserves in the heavenly dwellings the life of Elijah until the last day."
"After communion with the Divine Mysteries of the Body and Blood of Christ, Venerable Paisios sometimes remained without bodily food for seventy days; and there is nothing surprising in this, for divine grace has an indescribable power and therefore could support life in him better than refreshment with corruptible food. For those who live according to the carnal nature, the body, in order not to become exhausted, require to strengthen it with nutrition, and those who, like the incorporeal, have succeeded in a higher natural life, the divine power in abundance gives this grace, which human nature obeys, and lives no longer on bodily food, but on spiritual food. The Almighty Creator, our God, knows how to preserve the lives of seven sleeping youths on earth for three hundred years and even more. And indeed He preserves in the heavenly dwellings the life of Elijah until the last day."