How did He look like during His infancy, childhood, boyhood, youth and adulthood? Is there any mention of Him having an Ashtaratni body?
Lord Krishna's beauty is described wonderfully in Bhagavata Purana and is expounded further in Chaitanya Charitamrita.
O son of the king of the cowherds, Your transcendental body is dark blue like a new cloud, Your garment is brilliant like lightning, and the beauty of Your face is enhanced by Your gunja earrings and the peacock feather on Your head. Wearing garlands of various forest flowers and leaves, and equipped with a herding stick, a buffalo horn and a flute, You stand beautifully with a morsel of food in Your hand.(SB 10.14.1)
Krishna, is the reservoir of all beauty. All beautiful things emanate from Him, and His personal form is so attractive that it steals the eyes away from all other objects, which then seem devoid of beauty in comparison to Him. When Lord Krishna was on the earth, He attracted the eyes of all people. When he spoke, His words attracted the minds of all who remembered them. By seeing his footsteps, people became attracted to Him, and thus they wanted to offer their bodily activities to the Lord as His followers. In this way Krishna very easily spread His glories, which are sung throughout the world by the most sublime and essential Vedic verses. (SB 11.1.6–7)
Ornaments caress that body, but the transcendental body of Krsna is so beautiful that it beautifies the ornaments He wears. Therefore Krsna's body is said to be the ornament of ornaments. Enhancing the wonderful beauty of Krsna is His three-curved style of standing. Above all these beautiful features, Krsna's eyes dance and move obliquely, acting like arrows to pierce the minds of Gopis. When the arrow succeeds in hitting its target, their minds become agitated.(CC Madhya 21.105)
Dear Krsna, by seeing Your beautiful face decorated with tresses of hair, by seeing the beauty of Your earrings falling on Your cheeks, and by seeing the nectar of Your lips, the beauty of Your smiling glances, Your two arms, which assure complete fearlessness, and Your broad chest, whose beauty arouses conjugal attraction, we have simply surrendered ourselves to becoming Your maidservants.(CC Antya 15.70)