Here's a very simple explanation:

"have learned"... present perfect... learning started in the past and is still true right now... and it is possible that the learning might continue in the future

"have been learning"... present perfect continuous/progressive...  learning started in the past and is still continuing at this time.

Examples:

I have learned many things in my life... learning started in the past and and is still true right now... and it is possible that the learning might continue in the future (assuming I keep living)

I have been learning how to play the piano... learning started in the past and is still continuing at this time ("at this time" does not mean at the moment when I wrote "have been learning" BUT it suggests that the learning has not finished and I expect to continue learning in the future. (This would be true if I had a piano lesson every week and I am scheduled to have another lesson next week).

Summary:

The difference bewteen the two tenses is small, but the continuous tense is a strong indication that the learning will continue.

Let me know if you think my explanation is helpful...

Hey Loren,

"I have learned" is present perfect simple and it indicates that the action started in the past and has a result in the present, for example:

I have learned 20 new words! - you spent some time learning and now, in the present, you know some extra words

"I have been learning" is present perfect continuous and it indicates that the action started in the past as well, but here the process itself is more important than the result:

I have been learning new words all night! - you spent the entire night learning words but we don`t know if you know them now or not.

In the case of learning English you should say "I have been learning English since I was 20 years old" to emphasize that you started in the past and the process is still going on.

@Richard, I thank you very much.

Great explanation!!! Now it gets clear to me!!