This is an excerpt from the Srimad Bhagavatam, considered the quintessence of Vedic wisdom, which we read in my Ashram with my explanations.
Here it mentions how a Yogi breathed. Moreover, he was a boy.
By the way, here is something else about another boy Yogi -
here.
So, here is the excerpt itself:
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May good fortune accompany you, my boy. Go to the bank of the Yamuna, to the sacred forest of Madhuvan, to cleanse yourself of all impurities. Whoever comes to this place draws closer to the Lord, who eternally resides there.
Narada Muni continued: My boy, in the waters of the Yamuna, sometimes called Kalindi, you should bathe three times a day, since its waters are healing, sacred, and pure. After bathing, you should perform the exercises of ashtanga-yoga and then sit on an asana (a seat), assuming a motionless posture.
Having taken your seat on the asana, perform three types of breathing exercises. This will help you learn to control the life air, the mind, and the senses. Completely cleanse yourself of material impurities and, having cultivated patience, begin meditating on God.
O son of the king, now I shall reveal to you the mantra that should be repeated during meditation. Whoever repeats this mantra with concentration for seven nights will be able to see perfected beings flying through the sky.
Oṁ namo bhagavate vāsudevāya. This is a twelve-syllable mantra used to worship the Lord. One should install a murti, or material form of the Lord, and, while repeating this mantra, offer flowers, fruits, and other foods to the Deity in strict accordance with the instructions of the authorities and the sacred scriptures. One should perform Puja.
The Lord should be worshiped by offering Him pure water, garlands of fresh flowers, fruits, flowers, and vegetables growing in the forest, or by gathering fresh grass, flower buds, and even tree bark.
Having heard these instructions from the great sage Narada, Dhruva Maharaja, the king's son, circumambulated his spiritual teacher and respectfully bowed to him. Then he set out for the forest of Madhuvan.
Dhruva Maharaja arrived in Madhuvan and, after bathing in the Yamuna, began to fast. Without closing his eyes, he fasted with concentration until sunrise. After that, following the advice of the great sage Narada, he began worshiping God.
During the first month, Dhruva Maharaja subsisted only on fruits and berries. He ate once every three days to sustain his body, devoting all the rest of his time to worshiping God.
During the second month, Dhruva Maharaja ate once every six days, subsisting only on dry grass and leaves. Thus he continued worshiping the Lord.
During the third month, he stopped eating altogether and lived on water alone, drinking it once every nine days. He was constantly immersed in trance and worshiped the Lord, who is glorified in the finest verses.
During the fourth month, Dhruva Maharaja mastered his breathing exercises perfectly, taking one breath every twelve days. Thus he completely calmed his mind and immersed himself in meditation.
By the beginning of the fifth month, Maharaja Dhruva, the king's son, had learned to control his breathing so well that he could stand motionless on one leg like a pillar, fully concentrating his mind on Parabrahman.
He subdued his senses and their objects and thus focused his mind on the form of God, not allowing it to become distracted by anything else.
As Dhruva Maharaja, meditating in this way, took hold of the Lord, the master of all living beings, upon whom the entire material creation rests, the three worlds began to tremble.
After performing the breathing exercise, I felt so much new energy—it was like a new birth
Breathing technique: