I have a missionary friend who is well into her 70's. Though her husband died more than twenty years (in the country where they had served as missionaries - and was also buried there), she has not let that stop her. She can think of nothing else she wants to do than to continue pouring God's Truth and Love into the lives of the ones that God laid on her heart. So, she has.
One particular story was about a young man, now in his early 40's, who approached her and played the guessing game with her. "Do you know who I am?" Having had invested in the lives of literally hundreds of young people, she could not identify the now grown man. When he told her, she said that she almost fainted. She had known "Jackson" since he was five and he had been a handful. Even as a teenager, he got involved in gangs and dabbled in heavy recreational drugs. To most of us at that point, we may have classified him as a "goner". Not my missionary friend. And now Jackson is a pastor. To God be the glory!
Though my friend is older and somewhat slower - though I would be hard pressed to keep up with her - she refuses to quit. It is her heart's desire to die among her people just as her husband did. She never speaks of her age - other than a number.
What about you? Do you feel old? Useless? Achy and sickly? Weak and frail? Terminally ill? As I've said before, God never deems us useless in His work. That we do ourselves. What is He nudging you to do for Him today? He is fully aware of your circumstances (and your biological age). Think that it's impossible? Think again.