This is part of the story:
Keith met actor-director Clint Eastwood at a charity golf tournament in California in 2018. At the time, Eastwood was working on his film The Mule. Keith was surprised by the energy Eastwood seemed to have despite being 88 years old at the time. He asked Eastwood how he did it, at which point Eastwood remarked, "I don't let the old man in". Keith liked this line and immediately wrote the song, which is a ballad about a man dealing with his increasing age.