"Now Joshua was old and stricken in years; and the Lord said unto him, Thou art old and stricken in years, and there remaineth yet very much land to be possessed." (Joshua 13:1)
There is no set "retirement age" for the Christian, for there is always "yet very much land to be possessed." Joshua had survived 40 years in the wilderness, then led in the long hard conquest of Canaan, and was now at least 80 years of age. Not only was he "old and stricken in years," but God even told him he was old! But instead of allowing him to settle down to enjoy a few retirement years in his hard-won new home, God sent Joshua out once again for further conquests.
That must always be the case with those who love and serve the Lord. There is still much Scripture to study and learn, many people yet to reach with a gospel witness, many with whom to share God's love and comfort, much money yet to be earned to give to missions. Even those who must retire from active service or become confined at home still have much praying to accomplish.
No one who knows the redemptive love of Jesus Christ is ever too old to possess more "land" for the Lord. "The righteous shall flourish like the palm tree: he shall grow like a cedar in Lebanon. Those that be planted in the house of the Lord shall flourish in the courts of our God. They shall still bring forth fruit in old age; they shall be fat and flourishing" (Psalm 92:12-14).
Old age eventually comes to everyone who survives youth and middle age, but that does not mean it is time to quit. "O God, thou hast taught me from my youth: and hitherto have I declared thy wondrous works. Now also when I am old and greyheaded, O God, forsake me not; until I have shewed thy strength unto this generation, and thy power to every one that is to come" (Psalm 71:17-18). HMM