The answer is yes and no, but not in relation to development in the up to 25 yrs. I also want to point out that our brains aren't constantly changing - making new brain cells and new connections all our life. Unfortunately when we get really old, we don't replenish those brain cells and make new neuro transmitters as much. I agree with madisgram that once you're bipolar, you basically stay this way with the help of therapy and meds.

I think some forms of depression can get better as they learn better coping techniques and also circumstances change. There is one disorder called Schizotypal Personality Disorder and this can improve with lessening of symptoms one the person reaches 40-50 yrs old.