Usually acquired by men around the age of 40. It is not something like adrenaline that comes when you need it most, if you are above the age of 40 and reasonably healthy, you will get it. It can be used to do hard physical labor, or prove to the younger crowd that you are not as old as they think you are. It is usually lost around the ages of 65-70 depending on your health.
John has old man strength, so he could outwork even the healthiest young men in his profession.