Children are like wet cement. They are very moldable and able to conform to their surroundings. But, as they grow older, they begin to harden in their ways. There become hardened in their ways. At times, they reach a point, where nothing you do can change their behavior.
The obvious point of the illustration is this: be diligent with your children in their younger years to train them in the ways that they should go. This is the wisdom of Proverbs 19:18, “Discipline your son while there is hope.” If you lack the diligence when they are young, all hope may be lost as they harden in their ways.