I agree that maybe bipolar can go into remission but not outgrown. I don't think a chemical imbalance can be outgrown. I agree that maybe the symptoms may be put into remission once young adult life stressors are complete.
Although I had manic and depressive symptoms in my late teens and early 20's, I was not diagnosed with bipolar until I was 27. A fight with my then boyfriend is what triggered me getting help.