My former boss got his PhD at 79 years old and went back to work teaching leadership to the Eastern Orthodox church. That was 10 years ago, and he still is teaching now at 89. He was a Executive VP and head of our PC division back when we did the actual manufacturing for HP and Lenovo computers. I also know a doctor in my current town who is 92 years old and still practicing medicine as a licensed orthopedic surgeon, and one of my own doctors is 84 and is father still practices medicine in India at age 98.
It takes more to keep active and healthy in older age, but we are not doomed. My Mom is one person who thinks she is so old that she cannot do anything anymore so she doesn't try, but my nextdoor neighbor is 94 and still mows his own lawn, and last summer when we had a storm come through, he helped my 22 year old son cut down the tree with a chain saw. I'm not so blessed, but I'm long from giving up. I've been totally disabled and wheelchair ridden since 2015, and just last week submitted my application for seminary. I'm determined to become a hospital chaplain, Lord willing, despite my limitations, even if it is only part time and volunteer basis. As long as there is life in me, I will be working for the Lord til my time is done.
Not that this is possible for everyone, but with advancements in modern medicine, people are going a lot longer and doing a lot more than just 20 years ago, so my point is to encourage everyone not to give up. God can still use your talents for a long time to come.