Because Scripture tells us God created a man and communicated with him telling him to fill the eearth and subdue it.
Or do you think God laid a sperm and en egg on the ground and let nature take its course?
Why not an infant, who then grew into (I have reasons for saying so) an adolescent?
How old must a person be before such things can be communicated?
If I, as a child, understood those words when they were read to me from the Word, why could they not be understood by the one to whom they were said at a similar stage of life?