The white and grey matter does change as you age and has impact in your memory retention. But the learning part is effected by a different mechanism.
When you are a kid your brain is very flexible in the way it wires and rewires, making you very adapt at learning new things. As you age, the neutral connections in the brain become more and more strengthen, allowing you to do a number of tasks without much of a mental effort, something we usually call muscle memory. A downside to that is that your ability to learn new things go down.
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