Very young children are simply not set in their ways. They quickly assimilate what their parents or others around them signal to them. They would be molded by others either well or not so well.
This is why Solomon wrote this:
"Train up a child in the way he should go: and when he is old, he will not depart from it." Prov 22:6
Unfortunately the reverse may also be true:
[Train up a child wrongly and when he is old he will not depart from it.]
Jesus wants people to go back to the way they were as children before they became set in their ways of men. Then allow the Potter to rework the newly pliable clay:
"And the vessel that he made of clay was marred in the hand of the potter: so he made it again another vessel, as seemed good to the potter to make it.
Then the word of the LORD came to me, saying,
O house of Israel, cannot I do with you as this potter? saith the LORD. Behold, as the clay is in the potter's hand, so are ye in mine hand, O house of Israel." Jerem 18:4-6
The unformed molding clay is the very young child, still unspoiled by the evils of those around them who are already more experienced at evil. For God to work with an older person, the person must allow God to take him back to the unformed pliable clay of a child and reform him in the image of God.