This article, like other things I've read in books like "The Organized Mind", or the class "Learning How to Learn" is that learning for adults is usually difficult not because of reduced learning capacity, but instead individual behaviors, emotional regulation, and culture.

Dr. Levetin suggests that it's mostly around how much frustration you can handle, and how much time you think you have left vs. the amount of value you'll get from what you are learning. Procrastination, a great killer of self-education, is primarily driven by low emotional intelligence/regulation.

This is exactly what I get from this article again: the issue is not brain plasticity in learning, but the willingness to be frustrated, a desire to learn new things (we can't all be Paul Erdos) and the time to learn.