How do brain cells allow me to learn at 80?

Same way as before. And, in truth, we don't really know how learning occurs. We can point to the various structures in the brain, such as the hippocampus, which we know are part of the process. We know that memory can be placed anywhere in the cortex. These are known from studies of animals and humans who've sustained brain injuries. But what happens inside your brain when you form a memory remains a mystery.