Obedience to this command does not stop at a certain age. Genesis 48:12 reveals the deep respect Joseph had for Jacob when he brought his two sons before him for a blessing: "So Joseph brought them from beside his knees, and he bowed down with his face to the earth." With adulthood, the time may come when it is no longer necessary or right for a person to obey his parents strictly. But God's requirement to honor them never ceases. This duty pays dividends by giving us access to the wisdom of years.
Honor has wider application than obey. It expresses itself in courtesy, thoughtfulness, mercy, and kindly deeds. We would hardly consider one to be honoring his parents who, when they fall sick, weak, and perhaps blind in old age, does not exert himself to the utmost for them and their support in their need.
Just as surely as God requires parents to nourish, defend, support, and instruct the children in their lowest state of infancy, so children in their strength should support their parents in their weakness. Turn about is fair play because the Scripture says, "Whatever you want men to do to you, do also to them" (Matthew 7:12). Each of us would want someone to care for us in our time of need.