The problem of superman's steel body

Kryptonians were born more than 20 thousand years before us. Twenty thousand years later, when their Krypton was about to be destroyed, Superman's father sent Superman to earth. Because kryptonians were born more than 20,000 years before us, when Superman came to the earth, the talents on the earth had just begun to develop civilization, high technology, that is, modernity, so Superman came to the earth just in our time, so after he lived on the earth for 33 years, the villain General zod also came to the earth.