Will depression from 20 years ago prevent me from being an airline pilot?
It does not sound like a dealbreaker, however I'm not an aviation medical examiner. I do not think that depression diagnosed when you were a minor would prevent you from becoming a commercial pilot now, I do not even know if you would be required to declare it to an airline on an application. The biggest hurdle you would have is the medical I would expect. You might need to have a psychiatric evaluation, or it might be far back enough that isn't necessary. It likely depends on the severity of the depression you were diagnosed with on the DSM (diagnostic and statistical model of mental disorders, a tool used by professionals to assess and diagnose mental conditions) scale. 

My recommendation would be to talk to an aviation medical examiner. Many pilots deal with, or have dealt with depression, sometimes severe, so it should not be hard to answer. You lose nothing by trying.