The growth of children is irreversible. Just like a young tree, it takes the bends directed by the gardner, so is the life of a child.
You cannot shape it when it is adulthood, you can't pick it from where you left after you reach the top of your career pursuit.
Just like in the case of time, once spent, it becomes irreversible , it's gone forever.
Also in the case of a river, you cannot touch the same water twice while flowing