That's a really good question. I don't have an answer, and I sincerely doubt any of the literature out there defines a clear non-arbitrary line between the two.

Seems more important to ask the question: does this particular problem (whether it's bipolar disorder or teenage lows and highs) warrant psychiatric treatment? Psychiatrists usually are inclined to say 'yes,' regardless because both scenarios look the same to them.

The treatment is often fundamentally disabling, lousy at controlling the initial symptoms, and introduces a slew of new ones.

But because the term 'bipolar' currently equates to a drug therapy of antipsychotics/ moodstabilizers/antidepressants, it certainly shouldn't be tossed off lightly.

Counselling/Therapy would certainly appear to be a much safer/more effective road in any event.