Psalms 71:18, commentary and pictures

The verse is David's earnest prayer when old and grayheaded for God not to forsake him until he has shown God's strength to this generation and God's power to everyone to come, the plea of a faithful servant whose strength is failing to be kept to go on declaring God's strength and power to the present generation and to those yet to come.
He prayed this because he saw that most aged people around him were unhappy and that their unhappy traits increased with age, with those naturally close and covetous becoming disagreeably so and those jealous, fretful and impatient becoming especially so.
He was distressed to see kings and nobles who had seemed to fear God in the strength of manhood become jealous of their best friends and relatives when aged, suspecting selfish motives in them, listening to hints and deceptive advice from strangers, burning with unrestrained jealousy when others disagreed with their failing judgment, and growing dreadfully covetous in thinking their own children and relatives wished them dead to possess their wealth and honor, with some even controlled to destroy their own children.
He marked that although some had lived righteously in manhood they lost self-control in old age as Satan stepped in and guided their minds, making them restless and dissatisfied, seeming forsaken of God and exposed to the ridicule and reproaches of God's enemies.

Looking forward to his own old age, David was deeply moved and feared that God would leave him and he would become as unhappy as those aged persons and be left to the reproach of the enemies of the Lord, and he felt the necessity of guarding against the evils which attend old age.
God had taught him from his youth and hitherto he had declared God's wondrous works, showing God as the one who teaches from youth and remains worthy of trust in old age, here identified as Christ the divine Teacher.
Hezekiah's plea in his fatal illness echoes and applies this prayer, for after serving God and strengthening the people's confidence in Jehovah as their Supreme Ruler, he asked for help while near death from a desire to continue making God's strength and power known.
The verse joins dependence on God in old age with the duty to pass on a testimony of God's faithfulness and power, and David had served his own generation so that his experience, including both failures and lessons learned from God, became instruction for later generations.

Growing old does not lessen or end the call to serve God, for David's zeal remains undiminished and he asks God to stay with him rather than leave him to his own wisdom, so he can testify to the young people around him about God's works.
The verse expresses the continuing duty and desire of faithful older workers to keep bearing witness of God's power and truth to the present generation.
Growing old and gray does not release a person from service, for aged workers are to keep bearing their message as long as they have evidence that the Lord is helping them.