I really don't think there's much difference between learning as an adult or a teenager. It's just that we have less time. A few months ago I wanted to learn to appreciate and listen to classical music. I woke up an hour earlier every morning and read a book, listened to an audio course (Robert Greenberg's How to Listen to and Understand Great Music), and listened to actual music then and throughout the day. A few mo…
I'm mid 40's now. It is significantly different than when I was 20 or younger. It was like I had more horsepower back then. I could sit down and do 200 vocabulary words 90% retention after a single pass. I'm nowhere close now. On the other hand, I can organize my attack on learning something so much better than I could before.
Would be interesting to quantify such changes, but my guess would be that the changes are smaller than expected or felt.