Psalm 92:13-14:

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;

See also:

Don’t Get Sick When You Get Old
Selfish Dying and Satan-Assisted Suicide
Can I Believe to Become More Physically Attractive?
Objection: Ecclesiastes 12:1 Proves That You Should Expect to Get Sicker as You Age
Objection: David Didn’t Have His Youth Renewed – He Had to Have Abishag Keep Him Warm
Condition: Weakness